Core: add PRE/POST_DELETE_TOPIC events (#616).

2 new event listeners:
{{{
de.deepamehta.core.service.event.PreDeleteTopicListener
    void preDeleteTopic(Topic topic, Directives directives)

de.deepamehta.core.service.event.PostDeleteTopicListener
    void postDeleteTopic(Topic topic, Directives directives)
}}}

The core events are now complete orthogonality-wise.

Close #616.