StreamSource now have the option 'restart on deactivation' like MediaPlayer. This option is saved in XML (added Visitors for Stream and StreamSource). The GUI is added as sub-menu in play bar (like for MediaPlayer). Some StreamSource subclasses needed to be fixed to allow this feature (e.g. MultiFileSource).
When a source in a session fails, it is not anymore deleted after update; the Mixer keeps it in the session but detaches it from the scene. This way the user can access the failed source in the navigator (listed in RED), and Replace the source. The Replacement of source is available for any source. The source visitor does not skip a visit if the source failed.
