[IMP] web: modify notification.js like in enterprise

The enterprise web client had to be modified to fix a crash.  This crash
does not occur in the community web client,  but this commit makes sure
that the semantics of the notification manager is the same as the
enterprise version, to prevent future problems.

addons/web/static/src/js/widgets/notification.js

@@ -54,7 +54,8 @@ var NotificationManager = Widget.extend({
     className: 'o_notification_manager',
 
     display: function(notification) {
-        return notification.appendTo(this.$el);
+        notification.appendTo(this.$el);
+        return notification;
     },
     notify: function(title, text, sticky) {
         return this.display(new Notification(this, title, text, sticky));