#8 Support Vorbis discnumber and ID3 TPOS

[Hinrik Örn Sigurðsson]

Many albums come on multiple CDs (or tapes or whatever). The correctly tagging these means setting the ID3 TPOS frame and Vorbis 'discnumber' tag to the indicate which disc the track is a part of. It would be nice if tagtool had a "Disc" input box (below the "Track" one maybe) where one could set it.

Side note: The "Advanced" dialog for Vorbis doesn't even list the 'discnumber' tag. That needs to be fixed as well.