MythMusic: look for coverart in the Music/MusicArt storage groups

If the track is in a storage group and the coverart is in the same directory
as the track then grab the coverart from there.

Embedded coverart will be cached in the MusicArt storage group and loaded from
there but the BE side of things is not yet implemented.
Merge branch 'master' of code.mythtv.org:mythtv