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/overview.js

@@ -474,6 +474,10 @@ Overview.prototype = {
         this.emit('item-drag-begin');
     },
 
+    cancelledItemDrag: function(source) {
+        this.emit('item-drag-cancelled');
+    },
+
     endItemDrag: function(source) {
         this.emit('item-drag-end');
     },