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
Yes, but it was built on the model of other stuff (multiple lines, see `_display_info_tab`).
I'm thinking about adding more infos later in this dialog, i'll reduce code redundancy though.