Can we stop using MusicBrainz id as track uniqueid (was: [Bug 315329])

[Matěj Laitl]

Hi,
it has been brought to my attention on 
https://bugs.kde.org/show_bug.cgi?id=315329 that MusicBrainz changed semantics 
of the "MusicBrainz id" stored in various meta tags in a way that we IMO 
cannot use it as track unique identifier any longer.

In short, the "MusicBrainz id" is now a recording id, and per [1], one exact 
recording may easily end up in more albums, for example the original one and a 
compilation one. This breaks Amarok's assumption of uniqueness and leads to 
"Duplicate Tracks Found, only one imported" problems.

[1] https://wiki.musicbrainz.org/Server_Release_Notes/20110516

Is it okay if I just drop support for treating MusicBrainz id as uniqueIds? We 
may introduce MusicBrainz id as a separate tag in future. 
(Meta::valMusicBrainzId or similar)

	Matěj