[layouts] Add api to retrieve window from designer dialog interface

Author: nyalldawson

python/gui/auto_generated/layout/qgslayoutdesignerinterface.sip.in

@@ -61,6 +61,13 @@ Returns the current layout displayed in the designer.
 Returns the master layout displayed in the designer.
 
 .. seealso:: :py:func:`layout`
+%End
+
+    virtual QWidget *window() = 0;
+%Docstring
+Returns a pointer to the designer window.
+
+.. versionadded:: 3.4
 %End
 
     virtual QgsLayoutView *view() = 0;

src/app/layout/qgslayoutdesignerdialog.cpp

@@ -102,6 +102,11 @@ QgsAppLayoutDesignerInterface::QgsAppLayoutDesignerInterface( QgsLayoutDesignerD
   , mDesigner( dialog )
 {}
 
+QWidget *QgsAppLayoutDesignerInterface::window()
+{
+  return mDesigner;
+}
+
 QgsLayout *QgsAppLayoutDesignerInterface::layout()
 {
   return mDesigner->currentLayout();

src/app/layout/qgslayoutdesignerdialog.h

@@ -55,6 +55,7 @@ class QgsAppLayoutDesignerInterface : public QgsLayoutDesignerInterface
 
   public:
     QgsAppLayoutDesignerInterface( QgsLayoutDesignerDialog *dialog );
+    QWidget *window() override;
     QgsLayout *layout() override;
     QgsMasterLayoutInterface *masterLayout() override;
     QgsLayoutView *view() override;

src/gui/layout/qgslayoutdesignerinterface.h

@@ -85,6 +85,13 @@ class GUI_EXPORT QgsLayoutDesignerInterface: public QObject
      */
     virtual QgsMasterLayoutInterface *masterLayout() = 0;
 
+    /**
+     * Returns a pointer to the designer window.
+     *
+     * \since QGIS 3.4
+     */
+    virtual QWidget *window() = 0;
+
     /**
      * Returns the layout view utilized by the designer.
      * \see layout()