PICARD-658: Support loading the newly supported pregap and data tracks.

This adds initial support for the new pregap and data tracks. The tracks are loaded as part of the normal track list. To identify those tracks and e.g. deal with them differently in tagging the special metadata variables ~pregap and ~datatrack get set.

Fixes [PICARD-658](http://tickets.musicbrainz.org/browse/PICARD-658)

This is just a first most basic step, but I would like to get that merged. I have also some initial work in https://github.com/phw/picard/tree/ignore-special-tracks-for-completeness to fix [PICARD-514](http://tickets.musicbrainz.org/browse/PICARD-514) which will allow the user to ignore some types of special track types for the completeness of an album. I would like to update that code so it works with the new track types provided by the server and also add some kind of visual indicator for the track type.