Topicmaps API: client-side add_topicmap() (#656).

The client-side Topicmaps API provides an add_topicmap() method:
{{{
dm4c.get_plugin("de.deepamehta.topicmaps").add_topicmap(topicmap_id)
}}}

Plugins can call this method when they have created a topicmap (at server-side) and now want display it.

See #656