CAA types selection: replace free text input with a list with checkboxes

Free text input was simple, but it can't be translated, and isn't very user-friendly.
This patch introduces a CAA types selector, with translatable titles and descriptions.

Upgrade is seamless since the whole list is still saved/load as a simple string using
space as separator, with english lowercased names of each type.

Descriptions were taken from http://musicbrainz.org/doc/Cover_Art/Types

Define missing tabstops for Cover Art options UI
Regenerate picard/ui/ui_options_cover.py with pyuic 4.9.6
Merge remote-tracking branch 'origin/CAA_types_sel' into imagemeta
Translate type names in artwork tab of info dialog.