Override wxGrid::ScrollWindow() to scroll row/column labels too

Instead of doing it in overridden wxGridWindow::ScrollWindow(), do it
from wxGrid::ScrollWindow() itself, this makes more sense and will make
it easier to generalize it to scroll more windows.

No real changes yet.

include/wx/generic/grid.h

@@ -1068,6 +1068,8 @@ public:
     void DrawAllGridLines( wxDC& dc, const wxRegion & reg );
     void DrawCell( wxDC& dc, const wxGridCellCoords& );
     void DrawHighlight(wxDC& dc, const wxGridCellCoordsArray& cells);
+    void ScrollWindow( int dx, int dy, const wxRect *rect ) wxOVERRIDE;
+
     void UpdateGridWindows() const;
 
     // this function is called when the current cell highlight must be redrawn