[FEATURE] New standard widget for symbol buttons

Button widgets for configuring symbol properties were reimplemented
multiple times throughout the codebase. This commit creates a new
standard QgsSymbolButton widget which should be used whenever
a button for configuring symbol properties is required.

Features include:
- automatic use of inline panels whenever possible
- dropdown menu with shortcuts to color settings, copy/pasting colors
- accepts drag and dropped colors to set symbol color

python/gui/gui_auto.sip

@@ -188,6 +188,7 @@
 %Include qgsstatusbar.sip
 %Include qgssublayersdialog.sip
 %Include qgssubstitutionlistwidget.sip
+%Include qgssymbolbutton.sip
 %Include qgstablewidgetbase.sip
 %Include qgstabwidget.sip
 %Include qgstaskmanagerwidget.sip

python/gui/qgssymbolbutton.sip

@@ -0,0 +1,159 @@
+/************************************************************************
+ * This file has been generated automatically from                      *
+ *                                                                      *
+ * src/gui/qgssymbolbutton.h                                            *
+ *                                                                      *
+ * Do not edit manually ! Edit header and run scripts/sipify.pl again   *
+ ************************************************************************/
+
+
+
+class QgsSymbolButton : QToolButton
+{
+%Docstring
+ A button for creating and modifying QgsSymbol settings.
+
+ The button shows a preview icon for the current symbol, and will open a detailed symbol editor dialog (or
+ panel widget) when clicked.
+
+.. versionadded:: 3.0
+%End
+
+%TypeHeaderCode
+#include "qgssymbolbutton.h"
+%End
+  public:
+
+    QgsSymbolButton( QWidget *parent /TransferThis/ = 0, const QString &dialogTitle = QString() );
+%Docstring
+ Construct a new symbol button.
+ Use ``dialogTitle`` string to define the title to show in the symbol settings dialog.
+%End
+
+    virtual QSize minimumSizeHint() const;
+
+    void setDialogTitle( const QString &title );
+%Docstring
+ Sets the ``title`` for the symbol settings dialog window.
+.. seealso:: dialogTitle()
+%End
+
+    QString dialogTitle() const;
+%Docstring
+ Returns the title for the symbol settings dialog window.
+.. seealso:: setDialogTitle()
+ :rtype: str
+%End
+
+    QgsSymbol *symbol();
+%Docstring
+ Returns the current symbol defined by the button.
+.. seealso:: setSymbol()
+.. seealso:: changed()
+ :rtype: QgsSymbol
+%End
+
+    QgsMapCanvas *mapCanvas() const;
+%Docstring
+ Returns the map canvas associated with the widget.
+.. seealso:: setMapCanvas()
+ :rtype: QgsMapCanvas
+%End
+
+    void setMapCanvas( QgsMapCanvas *canvas );
+%Docstring
+ Sets a map ``canvas`` to associate with the widget. This allows the
+ widget to fetch current settings from the map canvas, such as current scale.
+.. seealso:: mapCanvas()
+%End
+
+    QgsVectorLayer *layer() const;
+%Docstring
+ Returns the layer associated with the widget.
+.. seealso:: setLayer()
+ :rtype: QgsVectorLayer
+%End
+
+    void setLayer( QgsVectorLayer *layer );
+%Docstring
+ Sets a ``layer`` to associate with the widget. This allows the
+ widget to setup layer related settings within the symbol settings dialog,
+ such as correctly populating data defined override buttons.
+.. seealso:: layer()
+%End
+
+    void registerExpressionContextGenerator( QgsExpressionContextGenerator *generator );
+%Docstring
+ Register an expression context generator class that will be used to retrieve
+ an expression context for the button when required.
+%End
+
+  public slots:
+
+    void setSymbol( QgsSymbol *symbol /Transfer/ );
+%Docstring
+ Sets the ``symbol`` for the button. Ownership of ``symbol`` is transferred to the
+ button.
+.. seealso:: symbol()
+.. seealso:: changed()
+%End
+
+    void setColor( const QColor &color );
+%Docstring
+ Sets the current ``color`` for the symbol. Will emit a changed() signal if the color is different
+ to the previous symbol color.
+%End
+
+    void copyColor();
+%Docstring
+ Copies the current symbol color to the clipboard.
+.. seealso:: pasteColor()
+%End
+
+    void pasteColor();
+%Docstring
+ Pastes a color from the clipboard to the symbol. If clipboard does not contain a valid
+ color or string representation of a color, then no change is applied.
+.. seealso:: copyColor()
+%End
+
+  signals:
+
+    void changed();
+%Docstring
+ Emitted when the symbol's settings are changed.
+.. seealso:: symbol()
+.. seealso:: setSymbol()
+%End
+
+  protected:
+
+    virtual void changeEvent( QEvent *e );
+
+    virtual void showEvent( QShowEvent *e );
+
+    virtual void resizeEvent( QResizeEvent *event );
+
+
+    virtual void mousePressEvent( QMouseEvent *e );
+
+    virtual void mouseMoveEvent( QMouseEvent *e );
+
+    virtual void dragEnterEvent( QDragEnterEvent *e );
+
+
+    virtual void dragLeaveEvent( QDragLeaveEvent *e );
+
+
+    virtual void dropEvent( QDropEvent *e );
+
+
+};
+
+/************************************************************************
+ * This file has been generated automatically from                      *
+ *                                                                      *
+ * src/gui/qgssymbolbutton.h                                            *
+ *                                                                      *
+ * Do not edit manually ! Edit header and run scripts/sipify.pl again   *
+ ************************************************************************/

src/gui/CMakeLists.txt

@@ -336,6 +336,7 @@ SET(QGIS_GUI_SRCS
   qgssubstitutionlistwidget.cpp
   qgssqlcomposerdialog.cpp
   qgsstatusbar.cpp
+  qgssymbolbutton.cpp
   qgstablewidgetbase.cpp
   qgstabwidget.cpp
   qgstablewidgetitem.cpp
@@ -492,6 +493,7 @@ SET(QGIS_GUI_MOC_HDRS
   qgsstatusbar.h
   qgssublayersdialog.h
   qgssubstitutionlistwidget.h
+  qgssymbolbutton.h
   qgstablewidgetbase.h
   qgstabwidget.h
   qgstaskmanagerwidget.h