Added a radio station context menu that allows to delete a radio station

Unfortunately, the Logitech squeezebox app does not refresh the radio
station list, so that the deleted station is still visible. Strangely,
it seems that adding a station to the favorites refreshes the list
automatically.
Added a track info menu provided by a track item subfeed.

Still problems with rating as the cache is only updated for the track and
not for albums, etc. which have also tracks not being updated.