dnd: Add a drag-cancelled signal

This lets us start the workspace zoom out animation right when the
snapback starts, rather than waiting for the snapback to finish.

https://bugzilla.gnome.org/show_bug.cgi?id=643786

js/ui/dnd.js

@@ -498,6 +498,7 @@ _Draggable.prototype = {
     },
 
     _cancelDrag: function(eventTime) {
+        this.emit('drag-cancelled', eventTime);
         this._dragInProgress = false;
         let [snapBackX, snapBackY, snapBackScale] = this._getRestoreLocation();