> Maybe registerNotificationHandler could give you a Registration object as result, where you then can do the unregister() ?

Good idea. the registerNotificationHandler() now returns a NotificationRegistration object with a unregister() method.
the AbstractModelControllerClient no longer keeps a map of its notification handlers this way.

I've not addressed the :reload way. I suppose this could be handled inside the NotificationRegistration object to re-register the notification handlers if a reload occurred. I'll address that in a separate issue.