Fix reading MusicBrainz disc ID according to MB spec
For ID3 GStreamer is reading (and writing) the MusicBranz disc ID from the non-standard TXXX:MusicBrainz DiscID frame, while MusicBrainz specifies and uses TXXX:MusicBrainz Disc Id.
This fixes at least the reading part by allowing both variations. IMHO we should also fix the writing part, but GStreamer itself seems to have set a precedence for this by using TXXX:MusicBrainz DiscID which then gets written by tools depending on GStreamer like e.g. SoundJuicer. Hence such files wildly exist now, which would be an argument to stick to what is currently done. Luckily not many tools actually do write disc ID tags at all