examples: Show how to hide dock indicators when ctrl is pressed

Fixes issue #334 and issue #337.
Won't add this directly into the library, as each project
has different requirements regarding this.

It's easy to do in application code though.

examples/dockwidgets/MyMainWindow.h

@@ -27,6 +27,7 @@ public:
 
 private:
     void createDockWidgets();
+    bool eventFilter(QObject *obj, QEvent *ev) override;
     KDDockWidgets::DockWidgetBase *newDockWidget();
     QMenu *m_toggleMenu = nullptr;
     const bool m_dockWidget0IsNonClosable;