Add helper wxGrid::DoAfterDraggingEnd() function

No real changes, just refactor the code to extract the part of
CancelMouseCapture() which can be useful not only when the mouse capture
is lost unexpectedly, but also when we release it of our own volition,
into a separate function.

include/wx/generic/grid.h

@@ -2218,6 +2218,10 @@ private:
     // SetColPos() and ResetColPos())
     void RefreshAfterColPosChange();
 
+    // reset the variables used during dragging operations after it ended
+    // because we lost mouse capture
+    void DoAfterDraggingEnd();
+
 
     // return the position (not index) of the column at the given logical pixel
     // position