[MPRIS] Extension of the spec with MusicBrainz tags

[Marius Orcsik]

>
> Yes, definitely.
>
> Please add them to the metadata guidelines on the freedesktop wiki page:
> https://www.freedesktop.org/wiki/Specifications/mpris-spec/metadata/
>
> You will need a freedesktop.org account for that.
>

Thank you for the reply Mirsal, it looks a bit convoluted to get a
freedesktop account, but I'll manage.

I'll ping back the mailing list once I've made the changes.

Cheers,
/Marius