overview: Handle drag-cancelled signal for items

Allow handling the drag-cancelled signal for non-window items, just
like the handling for windows instroduced in commit a80e88e33.

https://bugzilla.gnome.org/show_bug.cgi?id=644324

js/ui/appDisplay.js

@@ -412,6 +412,10 @@ AppWellIcon.prototype = {
                 this._removeMenuTimeout();
                 Main.overview.beginItemDrag(this);
             }));
+        this._draggable.connect('drag-cancelled', Lang.bind(this,
+            function () {
+                Main.overview.cancelledItemDrag(this);
+            }));
         this._draggable.connect('drag-end', Lang.bind(this,
             function () {
                Main.overview.endItemDrag(this);