Multiple disc albums show up as different albums

[Vasco]

When an album with multiple discs is added to the database (i.e., an album with the Disc Number tag filled out and for which there is more than one disc), then each disk is seen as a different album, both in the catalog view, and in the track view, so each disc shows up as a separate album under each artist.

They should instead show up as the same album with a divider between the tracks ordered first by disc and then by track number inside the disc.

[bflorat]

Dup of #1808