Get simple (text) drag and drop case working with drop source.

We ended up not needing the underlying QMimeData-based implementation, as the classes sitting on top of it
bypassed it anyway. It now treats the hierarchy as a pure data storage mechanism, generating the
QMimeData when drag-drop is initiated.

include/wx/qt/dnd.h

@@ -42,5 +42,8 @@ public:
                   const wxIcon &none = wxNullIcon);
     
     virtual wxDragResult DoDragDrop(int flags = wxDrag_CopyOnly);
+
+private:
+    wxWindow* m_parentWindow;
 };
 #endif // _WX_QT_DND_H_