MusicCommon: handle TrackUnavailableEvents from the music player

* This sets the font state to 'disabled' for the tracks item in the
  "currentplaylist" button list. (This should be a common font state used
   elsewhere in the Watch Recordings screen for example)

* Also sets the "playstate" displaystate of the item to "unavailable"

These changes should allow themers to mirror in the currentplaylist buttonlist
what appears for not available recordings on the  Watch Recording screen.