Topicmaps: adapt to Core changes pt.8 (#699).

Retrieving a topicmap including the topic's child child topics works again.
Needed for the Import Export plugin.

The "remove fetchComposite parameter" related work should be complete now. Ready for merging into master.

2 things remain to investigate (see "FIXME: had fetchComposite=true"):
    - Files API.
    - Create/Update requests containing topic references (see #614).
      In conjunction with Malte's Wikidata plugin.

See #699.