Webclient: change customizer methods (#505).

Changes in topicmap renderer customizer methods:

    - `modify_topic_view()` renamed to `update_topic()`. Additionally invoked when a generic TopicView is updated, that is its label and/or type has changed. The customizer is responsible for updating its depending properties accordingly.

    - `on_move_topic()` renamed to `move_topic()`.

See #505.