Split wxGrid RANGE_SELECT event into SELECTING and SELECTED

This will allow the applications that are only interested in the final
selection to ignore the intermediate SELECTING events, which are now
sent as soon as the selection changes while dragging the mouse, and only
handle the final SELECTED ones, when the drag is over.

include/wx/generic/gridsel.h

@@ -103,6 +103,9 @@ public:
 
     wxVectorGridBlockCoords& GetBlocks() { return m_selection; }
 
+    
+    void StartSelecting();
+    void EndSelecting();
 private:
     void SelectBlockNoEvent(const wxGridBlockCoords& block)
     {
@@ -138,6 +141,7 @@ private:
 
     wxGrid                              *m_grid;
     wxGrid::wxGridSelectionModes        m_selectionMode;
+    bool                                m_isSelecting;
 
     wxDECLARE_NO_COPY_CLASS(wxGridSelection);
 };