Support the performer:<type> tag

[Eric Beuque]

When I tag a song with MusicBrainz picard, some tracks have a "performer:<type>" tags defined.

There is few "types" like the "performer:vocal" tag. (http://musicbrainz.org/doc/Performer_Relationship_Type).

This is useful when you have a release from a composer, and the tracks with different singers.

For example, as you can see in this album (http://musicbrainz.org/release/e011ebcc-512c-44f1-b13d-786c18fa09a9.html) :
  - Album : Starmania (version originale)
  - Artist : Michel Berger & Luc Plamondon :
  - Track : 2 - Quand on arrive en ville => has vocal performed by Daniel Balavoine and Nanette Workman

It should be great if Exaile can support these tags (maybe with an option) :
  - With the search looking in these field (for example, if I look for "Balavoine", i want to find "Quand on arrive en ville")
  - And the tags values displayed in the collection after the track title.

Thank you for your software.