PICARD-227: Display read discid in Album info dialog

If a discid was read via CD Lookup feature, and associated to an album, display
the discid in the Album info dialog (from album's context menu).

http://tickets.musicbrainz.org/browse/PICARD-227
`info` will never have more than one element at this point, you don't need to map over it. In fact, it doesn't need to be a list in the first place.