PopupSwitchMenuItem: allow toggling without closing the menu

Similar to what Gtk does, now toggling with Space does not close
the menu.

https://bugzilla.gnome.org/show_bug.cgi?id=664416

js/ui/userMenu.js

@@ -613,7 +613,7 @@ const UserMenuButton = new Lang.Class({
         this._statusChooser = item;
 
         item = new PopupMenu.PopupSwitchMenuItem(_("Notifications"));
-        item.connect('activate', Lang.bind(this, this._updatePresenceStatus));
+        item.connect('toggled', Lang.bind(this, this._updatePresenceStatus));
         this.menu.addMenuItem(item);
         this._notificationsSwitch = item;