XDND: Fix dragMonitor leak in WorkspacesDisplay

WorkspacesDisplay removes its dragMonitor in _dragEnd, but
this was never called in when a xdnd drag ended causing
dragMonitors to stack up and handling events multiple times.

Fix that by making sure that _dragEnd is called when xdnd ends.

https://bugzilla.gnome.org/show_bug.cgi?id=644642

js/ui/overview.js

@@ -220,6 +220,7 @@ Overview.prototype = {
         this._resetWindowSwitchTimeout();
         this._lastHoveredWindow = null;
         DND.removeMonitor(this._dragMonitor);
+        this.endItemDrag();
     },
 
     _resetWindowSwitchTimeout: function() {