Topicmaps: fix display of initial topicmap (#520).

When the initial Webclient URL contained a topic reference dm4c.do_select_topic() was called twice. At time of the first call the topicmap might not yet be displayed (due to asynchronicity) which under some circumstances caused problems.

Now dm4c.do_select_topic() is called only once and at a time when the topicmap is guaranteed to be displayed.

See #520.