Refactor topicmap renderer framework, pt.4 (#505).

Hiding and deleting topics/association works.

BREAKING CHANGES

TopicmapRenderer interface:
    - remove_topic() replaced by hide_topic() and delete_topic()
    - remove_association() replaced by hide_association() and delete_association()

Remember: the topic renderer is responsible for updating the viewmodel now.

+++ NOT FULLY FUNCTIONAL +++

See #505.