Topicmap renderer refactoring, pt.9 (#505).

Polish TopicmapRenderer interface.

BREAKING CHANGES

2 methods in the TopicmapRenderer interface are renamed to better express their semantics:

    - add_topic()       -> show_topic()
    - add_association() -> show_association()

The TopicmapRenderer methods have no `refresh_canvas` parameter anymore. It is the responsibility of the renderer implementation (not the webclient's) to keep the display up-to-date.

    - update_topic(topic, refresh_canvas)          -> update_topic(topic)
    - update_association(topic, refresh_canvas)    -> update_association(topic)

    - hide_topic(topic_id, refresh_canvas)         -> hide_topic(topic_id)
    - hide_association(assoc_id, refresh_canvas)   -> hide_association(assoc_id)

    - delete_topic(topic_id, refresh_canvas)       -> delete_topic(topic_id)
    - delete_association(assoc_id, refresh_canvas) -> delete_association(assoc_id)

    - reset_selection(refresh_canvas)              -> reset_selection()

See #505.