Fix parsing of MusicBrainz data for discid

This pull request fixes #4025 where metadata gets scrambled when opening and audio-CD from the devices tab.

###### Fixes
* In case of a multi-disc release, fetch metadata for the correct disc.
* Fetch track numbers for a given discid. (#4025)
* Fix a bug where if an audio-CD is replaced by a CD which does not exist in the MusicBrainz database, the track list in the devices tab would retain the old metadata.

###### Changes
* In case of multiple artists, the artists are joined together using the MusicBrainz 'joinphrase'. Currently, only the last artist is chosen.


###### Other
* Add test for MusicBrainzClient.