Show cover art box by default.

Average user is expecting cover art to be shown in such application.
Download cover art by default, one front image only.

Average user is expecting cover art to be shown in such application.
Set default size for CAA images to 500px.
All providers are enabled so the behavior is the same than MusicBrainz website
(which shows cover art from (ie.) Amazon by default).
For CAA, download only approved images by default.
Set preferred releases to "Digital Media" and "[Worldwide]" by default.

If two releases with same tracklist exists, we want to take digital media over CD (ie),
because most users are tagging digital files from unknown source and don't care
which exact release it is until tracklist is matching.

Worldwide is preferred by default, because if two releases exist with same tracklist
it is more likely it is the worldwide one's than a country-specific one.
Merge pull request #213 from zas/defaults

Attempt to provide better defaults for the average user