Add wxGrid::RefreshBlock() helper

This simple function combines BlockToDeviceRect() and
wxWindow::Refresh() calls and allows to avoid the ugly casts in
wxGridSelection code as well as making the code slightly shorter and
more clear.

No real changes.

include/wx/generic/grid.h

@@ -1112,6 +1112,11 @@ public:
                          const wxArrayString& lines,
                          long *width, long *height ) const;
 
+    void RefreshBlock(const wxGridCellCoords& topLeft,
+                      const wxGridCellCoords& bottomRight);
+
+    void RefreshBlock(int topRow, int leftCol,
+                      int bottomRow, int rightCol);
 
     // ------
     // Code that does a lot of grid modification can be enclosed