[xwl] Cancel the visit when a drag ends

XWL only clears up when a "visit" finishes.

If we end a WlToXDrag through clicking we call "WlToXDrag::end" but the
visit object stays alive, which obviously will never finish.

Both the visit and the drag object then effectively leak and are used on
the next drag.

xwl/drag_wl.cpp

@@ -87,6 +87,7 @@ bool WlToXDrag::end()
         // we direclty allow to delete previous visits
         Q_EMIT finish(this);
     });
+    m_visit->leave();
     return false;
 }