IDs for non-MB sources

AlbumInfo and TrackInfo need MBID,s in their constructor. Different autotag-providers put different ID's in those places which they get from the sources(discogs got their ID, amazon's got ASIN) Finding a universal ID-system for all cds/lps/tracks/singles is next to impossible. EAN/UPC numbers are reasonably universal(except for 'old' material). So ...solution might be to move the MBID to a flexible attribute and allow MBID in constructor to be null.