Use the same selection expansion logic for Shift-Ctrl-cursor

Expanding the selection from keyboard with Ctrl pressed should move in
the same way Ctrl-cursor does, but use the same selection anchor as
Shift-cursor does instead of always using the current cell.

This makes the expansion work much more intuitively in the grid, e.g.
pressing Shift-Ctrl-Down in

    1 2
    3 4

grid when 1 and 2 are selected now selects all the cells instead of
selecting 1 and 3 as it did before.

include/wx/generic/grid.h

@@ -2767,6 +2767,12 @@ private:
                   wxGridWindow *gridWindow) const;
     int PosToEdgeOfLine(int pos, const wxGridOperations& oper) const;
 
+    // Fill the coords with the cell coordinates to use for the movement
+    // extending the current selection. Return false if, for whatever reason,
+    // we can't expand the selection at all.
+    bool PrepareForSelectionExpansion(wxGridCellCoords& coords,
+                                      const wxGridDirectionOperations& diroper);
+
     void DoMoveCursorFromKeyboard(const wxKeyboardState& kbdState,
                                   const wxGridDirectionOperations& diroper);
     bool DoMoveCursor(const wxKeyboardState& kbdState,