Core: add event POST_UPDATE_TOPIC_REQUEST (#587).

POST_UPDATE_TOPIC_REQUEST is fired once a topic update request (actually a `topic.update()` call) has performed.

Note: while `POST_UPDATE_TOPIC` is fired several times -- once for the request topic and, in case of a composite topic, once per child topic -- `POST_UPDATE_TOPIC_REQUEST` on the other hand is fired only once (for the request topic).

Listener interface:
{{{
import de.deepamehta.core.service.event.PostUpdateTopicRequestListener
}}}

Listener method:
{{{
void postUpdateTopicRequest(Topic topic)
}}}

See #587.