disc number support

[Vic333]

Surprised to see that Subsonic doesn't have proper disc number support. Odd. Hope this basic piece of functionality can be implemented.

[acroyear]

If the disc number is already there, it will use it when sorting the results of a folder/album*. It is returned in every individual song listing (whether from playlist, music directory, album, or search/random songs). All the system doesn't do is allow the ability to edit it through the tag editor...but again, if disc # is already there it will sort that page by it as well.

* this is as of 6.0beta1, I think. Older versions did ignore the disc # in sorting though it did return it in a song's entry.