[FEATURE] Reporting framework

Reports are based on the new layouts engine. They consist of multiple
nested sections. Each individual section (and the report itself)
can have an optional header and footer (which are themselves layouts,
and can consist of multiple pages!).

Two different types of sections are implemented so far:
- a standard section, which has a single, static body layout. This
can be used to embed static layouts mid way through a report
- a "field group" section, which repeats its body layout for
every feature in a layer. The features are sorted by the selected
grouping feature (with an option for ascending/descending sort).
If a field group section has child sections (e.g. another field
group section with a different field, then only features
with unique values for the group feature are iterated over.
This allows nested reports, e.g.

Report
- Country: Australia
    - State: NSW
        - Town: Sydney
        - Town: Woolongong
    - State: QLD
        - Town: Beerburrum
        - Town: Brisbane
        - Town: Emerald
- Country: NZ
    - State: ... etc

In this example country, state or town groups can have their
own headers and footers which will be inserted in the report.

Reports are configured through a new panel in the layout designer
dialog, which is shown when editing a report (created through
the Layout Manager Dialog). The organizer allows for adding
(and removing) sections to the report, and for selecting which
layout (e.g. headers, footers, bodies) to edit within the
layout designer.

python/core/core_auto.sip

@@ -162,6 +162,7 @@
 %Include composer/qgscomposertexttable.sip
 %Include composer/qgspaperitem.sip
 %Include layout/qgsabstractlayoutiterator.sip
+%Include layout/qgsabstractreportsection.sip
 %Include layout/qgslayoutaligner.sip
 %Include layout/qgslayoutexporter.sip
 %Include layout/qgslayoutgridsettings.sip

python/core/layout/qgsabstractreportsection.sip

@@ -0,0 +1,270 @@
+/************************************************************************
+ * This file has been generated automatically from                      *
+ *                                                                      *
+ * src/core/layout/qgsabstractreportsection.h                           *
+ *                                                                      *
+ * Do not edit manually ! Edit header and run scripts/sipify.pl again   *
+ ************************************************************************/
+
+
+class QgsAbstractReportSection : QgsAbstractLayoutIterator
+{
+%Docstring
+ An abstract base class for QgsReport subsections.
+
+.. versionadded:: 3.0
+%End
+
+%TypeHeaderCode
+#include "qgsabstractreportsection.h"
+%End
+  public:
+
+    QgsAbstractReportSection();
+%Docstring
+Constructor for QgsAbstractReportSection
+%End
+
+    ~QgsAbstractReportSection();
+
+
+
+    virtual QgsAbstractReportSection *clone() const = 0 /Factory/;
+%Docstring
+Clones the report section. Ownership of the returned section is
+transferred to the caller.
+
+Subclasses should call copyCommonProperties() in their clone()
+implementations.
+%End
+
+
+    virtual QgsLayout *layout();
+
+    virtual bool beginRender();
+
+    virtual bool next();
+
+    virtual bool endRender();
+
+
+    bool headerEnabled() const;
+%Docstring
+Returns true if the header for the section is enabled.
+
+.. seealso:: :py:func:`setHeaderEnabled()`
+
+.. seealso:: :py:func:`header()`
+
+.. seealso:: :py:func:`setHeader()`
+%End
+
+    void setHeaderEnabled( bool enabled );
+%Docstring
+Sets whether the header for the section is ``enabled``.
+
+.. seealso:: :py:func:`headerEnabled()`
+
+.. seealso:: :py:func:`header()`
+
+.. seealso:: :py:func:`setHeader()`
+%End
+
+    QgsLayout *header();
+%Docstring
+Returns the header for the section. Note that the header is only
+included if headerEnabled() is true.
+
+.. seealso:: :py:func:`setHeaderEnabled()`
+
+.. seealso:: :py:func:`headerEnabled()`
+
+.. seealso:: :py:func:`setHeader()`
+%End
+
+    void setHeader( QgsLayout *header /Transfer/ );
+%Docstring
+Sets the ``header`` for the section. Note that the header is only
+included if headerEnabled() is true. Ownership of ``header``
+is transferred to the report section.
+
+.. seealso:: :py:func:`setHeaderEnabled()`
+
+.. seealso:: :py:func:`headerEnabled()`
+
+.. seealso:: :py:func:`header()`
+%End
+
+    bool footerEnabled() const;
+%Docstring
+Returns true if the footer for the section is enabled.
+
+.. seealso:: :py:func:`setFooterEnabled()`
+
+.. seealso:: :py:func:`footer()`
+
+.. seealso:: :py:func:`setFooter()`
+%End
+
+    void setFooterEnabled( bool enabled );
+%Docstring
+Sets whether the footer for the section is ``enabled``.
+
+.. seealso:: :py:func:`footerEnabled()`
+
+.. seealso:: :py:func:`footer()`
+
+.. seealso:: :py:func:`setFooter()`
+%End
+
+    QgsLayout *footer();
+%Docstring
+Returns the footer for the section. Note that the footer is only
+included if footerEnabled() is true.
+
+.. seealso:: :py:func:`setFooterEnabled()`
+
+.. seealso:: :py:func:`footerEnabled()`
+
+.. seealso:: :py:func:`setFooter()`
+%End
+
+    void setFooter( QgsLayout *footer /Transfer/ );
+%Docstring
+Sets the ``footer`` for the section. Note that the footer is only
+included if footerEnabled() is true. Ownership of ``footer``
+is transferred to the report section.
+
+.. seealso:: :py:func:`setFooterEnabled()`
+
+.. seealso:: :py:func:`footerEnabled()`
+
+.. seealso:: :py:func:`footer()`
+%End
+
+    int childCount() const;
+%Docstring
+Return the number of child sections for this report section. The child
+sections form the body of the report section.
+
+.. seealso:: :py:func:`children()`
+%End
+
+    QList< QgsAbstractReportSection * > children();
+%Docstring
+Return all child sections for this report section. The child
+sections form the body of the report section.
+
+.. seealso:: :py:func:`childCount()`
+
+.. seealso:: :py:func:`child()`
+
+.. seealso:: :py:func:`appendChild()`
+
+.. seealso:: :py:func:`insertChild()`
+
+.. seealso:: :py:func:`removeChild()`
+%End
+
+    QgsAbstractReportSection *child( int index );
+%Docstring
+Returns the child section at the specified ``index``.
+
+.. seealso:: :py:func:`children()`
+%End
+
+    void appendChild( QgsAbstractReportSection *section /Transfer/ );
+%Docstring
+Adds a child ``section``, transferring ownership of the section to this section.
+
+.. seealso:: :py:func:`children()`
+
+.. seealso:: :py:func:`insertChild()`
+%End
+
+    void insertChild( int index, QgsAbstractReportSection *section /Transfer/ );
+%Docstring
+Inserts a child ``section`` at the specified ``index``, transferring ownership of the section to this section.
+
+.. seealso:: :py:func:`children()`
+
+.. seealso:: :py:func:`appendChild()`
+%End
+
+    void removeChild( QgsAbstractReportSection *section );
+%Docstring
+Removes a child ``section``, deleting it.
+
+.. seealso:: :py:func:`children()`
+%End
+
+    void removeChildAt( int index );
+%Docstring
+Removes the child section at the specified ``index``, deleting it.
+
+.. seealso:: :py:func:`children()`
+%End
+
+  protected:
+
+    enum SubSection
+    {
+      Header,
+      Body,
+      Footer,
+      End,
+    };
+
+    void copyCommonProperties( QgsAbstractReportSection *destination ) const;
+%Docstring
+Copies the common properties of a report section to a ``destination`` section.
+This method should be called from clone() implementations.
+%End
+
+  private:
+    QgsAbstractReportSection( const QgsAbstractReportSection &other );
+};
+
+
+class QgsReport : QgsAbstractReportSection
+{
+%Docstring
+ Represents a report for use with the QgsLayout engine.
+
+Reports consist of multiple sections, represented by QgsAbstractReportSection
+subclasses.
+
+.. versionadded:: 3.0
+%End
+
+%TypeHeaderCode
+#include "qgsabstractreportsection.h"
+%End
+  public:
+
+    QgsReport();
+%Docstring
+Constructor for QgsReport.
+%End
+
+    virtual QgsReport *clone() const;
+
+
+    virtual int count();
+    virtual bool beginRender();
+
+    virtual bool next();
+
+
+    virtual QString filePath( const QString &baseFilePath, const QString &extension );
+
+
+};
+
+/************************************************************************
+ * This file has been generated automatically from                      *
+ *                                                                      *
+ * src/core/layout/qgsabstractreportsection.h                           *
+ *                                                                      *
+ * Do not edit manually ! Edit header and run scripts/sipify.pl again   *
+ ************************************************************************/

src/core/CMakeLists.txt

@@ -364,6 +364,7 @@ SET(QGIS_CORE_SRCS
   dxf/qgsdxfpaintengine.cpp
   dxf/qgsdxfpallabeling.cpp
 
+  layout/qgsabstractreportsection.cpp
   layout/qgslayout.cpp
   layout/qgslayoutaligner.cpp
   layout/qgslayoutatlas.cpp
@@ -1029,6 +1030,7 @@ SET(QGIS_CORE_HDRS
   composer/qgspaperitem.h
 
   layout/qgsabstractlayoutiterator.h
+  layout/qgsabstractreportsection.h
   layout/qgslayoutaligner.h
   layout/qgslayoutexporter.h
   layout/qgslayoutgridsettings.h