grabHelper: Ungrab the entire stack on "outside clicks"

Currently clicks outside the grabbed actors are handled the same as
the user pressing Escape - a single actor is popped from the grab stack.
However according to the design, outside clicks should release all grabs.

https://bugzilla.gnome.org/show_bug.cgi?id=682243

js/ui/grabHelper.js

@@ -330,7 +330,7 @@ const GrabHelper = new Lang.Class({
             // which should be a release event.
             if (press)
                 this._ignoreRelease = true;
-            this.ungrab();
+            this.ungrab({ actor: this._grabStack[0].actor });
         }
 
         return this._modalCount > 0;