Add wxGridSelection::SelectAll() and use it in wxGrid

The difference between calling SelectAll() and SelectBlock() with a
block covering the entire grid is that the former discards any
previously selected blocks, which become clearly redundant.

As a consequence, clicking on the grid corner 10 times in a row still
results in a selection with a single block, not 10 (identical) blocks.

include/wx/generic/gridsel.h

@@ -52,6 +52,10 @@ public:
                     kbd, sendEvent);
     }
 
+    // This function replaces all the existing selected blocks (which become
+    // redundant) with a single block covering the entire grid.
+    void SelectAll();
+
     void DeselectBlock(const wxGridBlockCoords& block,
                        const wxKeyboardState& kbd = wxKeyboardState(),
                        bool sendEvent = true );