j.u.EventListener should really be tied to j.u.EventObject, which is useful for our purposes. First of all it does not really specify a contract. Second of all EventObject is a serializable class which means it cannot be an interface.

Eliminate all references to j.u.EventListener, so that we do not imply it. This impacts concepts.ListenerRegistration the most, as without EventListener it loses any semantic distinction from concepts.ObjectRegistration. Therefore ListenerRegistration should be kept as is and be deprecated for removal instead.

While at it, also remove Registration specializations which do not bring anything to the table.