Make MusicBrainz tagger more accurate and easy

The move to web service 2 makes disc number and artist credit better
defined, and multiple artists are not a problem anymore. Many more
results are now shown, with the new "Choose Best Matches from This
Album" action to avoid mixing tags from different albums.

Some speedups and visual/usability improvements complete the patch.

REVIEW: 105290
GUI: MusicBrainz tagger: more tags shown, new toolbar, and an improved
context menu for each search result (with >1 tracks only).
CCMAIL:
DIGEST: Amarok's MusicBrainz tagger can now help choose the best tags possible.