audio: Ensure tests can be run on their own
audio: Nest tags in scan data return value
audio: Change audio data convert to operate on taglists
models: Make .copy(foo=None) null out field.
audio: Update how translator test data is built.
audio: Add tests for all fields that can be converted
model: Update to handle None in sets.

Also adds some missing tests for composers and performers.
audio: Cleanup translator code.

This code was trying to be to smart for it's own good. This commit greatly
reduces the magic and leaves us with much more straight forward code.
audio: Review fixes
Merge pull request #644 from adamcik/feature/scanner-tag-handling

Cleanup taglist handling and translation to tracks.