FetchArt : fix an error with CAA where the pre-sized thumbs of release groups would be ignored

[AdrienCos]

Description

This patch fixes two bugs with the Cover Art Archive provider of the FetchArt plugin:

• the pre-sized thumbnails of the release group of an album would be ignored when the maxwidth option matches one of the available sizes (250, 500, and 1200 pixels).
  • This allows the release group to behave exactly like the release itself (which does take the pre-sized thumbnails into account).
• no cover would be found and downloaded when the maxsize option is defined, is in the pre-sized thumbnails list, but CAA does not provide thumbnails for this release/release group

This patch can significantly speed up the fetching process of some albums, as the default size of some cover arts can be quite large (e.g. this cover is 56 MB and 7300 pixels wide, while the 1200px version is 450KB).

This patch also add a section in the FetchArt plugin documentation, detailing this behavior.

To Do

• Documentation. (If you've add a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst near the top of the document.)
• Tests. (Encouraged but not strictly required.)

[sampsyo]

Awesome!! This looks really handy. Thanks for the clean fix, and for extending the documentation to match. When/if the CI goes green, this looks ready to merge.