Make interface between wxGridHeaderCtrl and wxGrid more explicit

Rename the functions used from wxGridHeaderCtrl event handlers to start
with DoHeader prefix to make it clear that they're (only) used by it in
an attempt to make things more clear and more uniform.

No real changes.

include/wx/generic/private/grid.h

@@ -252,24 +252,19 @@ private:
 
     void OnBeginResize(wxHeaderCtrlEvent& event)
     {
-        GetOwner()->DoStartResizeCol(event.GetColumn());
+        GetOwner()->DoHeaderStartDragResizeCol(event.GetColumn());
 
         event.Skip();
     }
 
     void OnResizing(wxHeaderCtrlEvent& event)
     {
-        GetOwner()->DoUpdateResizeColWidth(event.GetWidth());
+        GetOwner()->DoHeaderDragResizeCol(event.GetWidth());
     }
 
     void OnEndResize(wxHeaderCtrlEvent& event)
     {
-        // we again need to pass a mouse event to be used for the grid event
-        // generation but we don't have it here so use a dummy one as in
-        // UpdateColumnVisibility()
-        wxMouseEvent e;
-        e.SetState(wxGetMouseState());
-        GetOwner()->DoEndDragResizeCol(e);
+        GetOwner()->DoHeaderEndDragResizeCol(event.GetWidth());
 
         event.Skip();
     }