public void addListener( @Nonnull JID entity, @Nonnull EntityCapabilitiesListener listener ) {
        userSpecificCapabilitiesListener.put( entity, listener );
    }
Registers an event listener that will be invoked when the detected entity capabilities for a particular entity
have changed.

This method supports multiple event listeners per JID. Registration of the same combination of entity and listener
will be ignored.

@param entity The entity for which to listen for events.
@param listener The event listener to be invoked when entity capabilities have changed.
xmppserver/src/main/java/org/jivesoftware/openfire/entitycaps/EntityCapabilitiesManager.java