Implement wxFrame::DoSetClientSize()

Resize the central widget to fit the new size.

See https://github.com/wxWidgets/wxWidgets/pull/1340

include/wx/qt/frame.h

@@ -56,6 +56,7 @@ public:
 
 protected:
     virtual void DoGetClientSize(int *width, int *height) const wxOVERRIDE;
+    virtual void DoSetClientSize(int width, int height) wxOVERRIDE;
 
     virtual QWidget* QtGetParentWidget() const wxOVERRIDE;