[Notifications] Support persistency

This allows a client to mark a notification as "resident" which means it will
not be closed by the sever when expired or an action is invoked, thus transferring
full control over notification life time to the client.

A notification popup still times out as normal (unless persistent, of course)
but the notification remains alive until the client revokes it or a user
explicitly closes it.

It is opt-in so that a client can expect the notification lifetime to be
indefinite and explicitly close it when an action is invoked on its end.