Fix double click with DOM based renderers (#505).

View Customization Framwork:
The Webclient's "topic_doubleclicked" event is fired for DOM based renderers as well.

E.g. double clicking a File topic works (the file is opened in its origin application).

BREAKING CHANGE

A domain topic (a Topic object) is passed to the topic_doubleclicked handler (not a ViewTopic object).

See #505.