message: Restrict access to connect/disconnect for `notificationClicked()`
diff: @@ -45,7 +45,7 @@ public:
  void push(Notification notification);
  void clear();
  - NotificationCallback& notificationClicked() { return mNotificationClicked; }
  + NotificationCallback::Source& notificationClicked() { return mNotificationClicked; }
  void update() override;
  