Send the autosize column event for grid native header columns

Send wxEVT_GRID_COL_AUTO_SIZE on double clicking on a separator line of
the grid native header to allow override default behaviour.

include/wx/generic/grid.h

@@ -2404,6 +2404,8 @@ private:
                                    wxGridColLabelWindow* colLabelWin);
     void ProcessCornerLabelMouseEvent(wxMouseEvent& event);
 
+    void HandleColumnAutosize(int col, const wxMouseEvent& event);
+
     void DoColHeaderClick(int col);
 
     void DoStartResizeCol(int col);