Use dedicated event type to notify header about column width changes

Notification about changes of column widths needs to be sent locally from wxPropertyGrid to  wxPropertyGridManager (to update the header) so it would be good to use a dedicated non-public event type for these purposes.

include/wx/propgrid/manager.h

@@ -543,8 +543,8 @@ protected:
 #endif
     void OnResize( wxSizeEvent& event );
     void OnPropertyGridSelect( wxPropertyGridEvent& event );
-    void OnPGColDrag( wxPropertyGridEvent& event );
     void OnPGScrollH(wxPropertyGridEvent& evt);
+    void OnColWidthsChanged(wxPropertyGridEvent& evt);
 
 
     wxPropertyGrid* m_pPropGrid;