* A module can override the method "hidden" to specify when all it's
widgets should be hidden.
* Implement hidden for cmus and spotify
* Fix problem that cmus widgets were not displayed correctly
fixes#157
OK - I admit it: Mostly for the benefit of Travis and automated testing,
which complains about the DBusException, move to a more generic
exception.
However, this is probably a good idea anyhow, because independently of
the error, setting the song to an empty string is probably the best bet.
