Major Description
Currently the only notification that a user session has gone is org.apache.wicket.Application#sessionUnbound(String) which gives only the http session id as context.
With the proposed change it will be possible to have an explicit callback method in o.a.w.Session itself. It will be called after explicit usage of Session#invalidate(Now)() or http session expiration due to inactivity.
See the proposed patch.
One "problem" is that now the http session attribute that holds the instance of SessionBindingListener will keep a reference to the Wicket session, but I think this should not cause bigger memory footprint because the same Session instance is already kept in another attribute in the http session, so the serialization process will just link the second to the first.