JMPDComm: Move the unknown translations back to JMPDComm.

Move the unknown metadata translations back into JMPDComm using standard Java
internationalization.
Music: If albumartist or artist is not available, use unknown artist.
ignore case when comparing Strings for Music sorting
don't sort Music by disc and track number for searches, but everywhere else
fix String comparison
fix Album Artist not showing on SongsFragment

fixes #626
Add rudimentary support for showing/setting rating of current song.

Ratings range from 0 to 10 and are displayed as 0 to 5 stars with
1/2 star resolution.

External changes are not yet noticed.

Layout is... bad.