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.

Define missing tabstops for Cover Art options UI
Regenerate picard/ui/ui_options_cover.py with pyuic 4.9.6
Tiny loop simplification
Introduce CoverArtDownloader class

coverart.py cleanup on the way
CoverArtDownloader: step 2
CoverArtDownloader: step 3
Pass CoverArtDownloader object instead of try_list
Store album, release, metadata in CoverArtDownloader object
_fill_try_list(): drop album parameter, use cover_art_downloader property instead
_fill_try_list(): drop release parameter
_walk_try_list(): drop album, metadata, release parameters
_coverart_downloaded(): drop album, metadata, release parameters
_caa_json_downloaded(): drop album, metadata, release parameters
Move CoverArtDownloader declaration on top of the file
Move COVERART_SITES to class CoverArtDownloader
Move AMAZON_SERVER to class CoverArtDownloader
Move AMAZON_IMAGE_PATH to class CoverArtDownloader
Move AMAZON_ASIN_URL_REGEX to class CoverArtDownloader
Move _CAA_THUMBNAIL_SIZE_MAP to class CoverArtDownloader
_try_list_append_image_url() becomes a CoverArtDownloader method
_process_asin_relation() becomes a CoverArtDownloader method