flutter: Add convenience to access the controller

Dart can't access the DockWidgetViewInterface since Dartagnan
doesn't support multiple inheritance

src/flutter/views/DockWidget_flutter.cpp

@@ -73,3 +73,8 @@ QSize DockWidget_flutter::maxSizeHint() const
 
     return View_flutter::maxSizeHint();
 }
+
+Controllers::DockWidget *DockWidget_flutter::dockWidget() const
+{
+    return m_dockWidget;
+}

src/flutter/views/DockWidget_flutter.h

@@ -63,6 +63,8 @@ public:
     /// @reimp
     QSize maxSizeHint() const override;
 
+    Controllers::DockWidget *dockWidget() const;
+
 protected:
     void init() override;
 };