Make Reparent() work for wxFrame in wxQt

Use the correct parent window, i.e. the one of the central widget and
not of the frame itself.

include/wx/qt/frame.h

@@ -57,6 +57,8 @@ public:
 protected:
     virtual void DoGetClientSize(int *width, int *height) const wxOVERRIDE;
 
+    virtual QWidget* QtGetParentWidget() const wxOVERRIDE;
+
 private:
     // Common part of all ctors.
     void Init()