grabHelper: Consider events that release the grab handled

Currently, if a button-press event results in releasing the last modal
grab (e.g. clicks outside the grabbed actors), we don't consider the
event handled and allow its emission to continue. If we consider
dismissing a grab as an action of its own, any additional action
triggered by the same event becomes an unexpected side effect.
Tweak the capture handler accordingly.

https://bugzilla.gnome.org/show_bug.cgi?id=696422

js/ui/grabHelper.js

@@ -355,6 +355,7 @@ const GrabHelper = new Lang.Class({
                 this._ignoreRelease = true;
             let i = this._actorInGrabStack(event.get_source()) + 1;
             this.ungrab({ actor: this._grabStack[i].actor, isUser: true });
+            return true;
         }
 
         return this._modalCount > 0;