Save MusicBrainz DiscID to tags.

Probably needs some tests for this and also some more testing than what I've done (ie., I've just tested that it works on my system).

Rationale:
We already calculate the DiscID for looking up against the MusicBrainz database and currently just "throw it away" afterwards. Since morituri ejects the media, I currently have to re-insert it and tell Picard to lookup the disc if I want to have the DiscID saved to the files. (If I open up Picard by giving it the files directly, I also have to deassociate the files with the release from the MBID and remove the release from Picard's view to enable it to include the DiscID in its release information.)