Add wxGrid::m_dragMoveCol field separate from m_dragRowOrCol

Don't reuse the same m_dragRowOrCol variable for both the index of the
row or column being drag-resized and for the index of the column being
drag-moved. Reusing it was confusing and made it more difficult what the
code was doing and what invariants were preserved in it, and just wasn't
worth saving a few bytes per wxGrid object.

No real changes.

include/wx/generic/grid.h

@@ -2298,6 +2298,10 @@ protected:
     bool    m_canDragGridSize;
     bool    m_canDragCell;
 
+    // Index of the column being drag-moved or -1 if there is no move operation
+    // in progress.
+    int     m_dragMoveCol;
+
     // the last position (horizontal or vertical depending on whether the user
     // is resizing a column or a row) where a row or column separator line was
     // dragged by the user or -1 of there is no drag operation in progress