Extend current selection on {Ctrl,Shift}-Space

Extend the existing selection instead of adding a new block, as there is
no reason to keep the old selection as a separate block, when it's
always a subblock of the new one, and doing it resulted in wrong
behaviour when selecting 2 horizontally adjacent cells, pressing
Ctrl-Space and then pressing Shift-Left deselected the rightmost
selected column but still left its single cell, which was part of the
originally selected block, selected, which was surprising and looked
wrong.

src/generic/grid.cpp

@@ -5872,7 +5872,7 @@ void wxGrid::OnKeyDown( wxKeyEvent& event )
                     }
 
                     if ( selStart != wxGridNoCellCoords )
-                        m_selection->SelectBlock(selStart, selEnd);
+                        m_selection->ExtendCurrentBlock(selStart, selEnd, event);
                 }
                 break;