Refactor topicmap renderer framework, pt.7 (#505).

Move certain cluster code from view to viewmodel.

BREAKING CHANGES

Topicmaps plugin: rename REST client methods
    dm4c.restc.move_topic()   -> dm4c.restc.set_topic_position()
    dm4c.restc.move_cluster() -> dm4c.restc.set_cluster_position()

See #505.