Make wxAUI and wxSplitterWindow work properly with wxOverlay under wxGTK2

This Workaround is needed because the DC used to draw on the wxOverlay clips
children by default under wxGTK2. This means that the sash will always appear
behind the sub-windows. making wxOverlay worthless under this platforms.

Notice that calling gdk_gc_set_subwindow() unconditionally in Destroy() is cheap
when DontClipSubWindows() has never been called.

include/wx/gtk/dcclient.h

@@ -122,6 +122,7 @@ public:
     PangoFontDescription *m_fontdesc;
 
     void SetUpDC( bool ismem = false );
+    void DontClipSubWindows();
     void Destroy();
 
     virtual void ComputeScaleAndOrigin() override;