This commit adds support for loading cover art from media file
directories, according to configured filename priorities (of which an
additional, special choice of `embedded` is given).
Cover art paths are resolved during scanning and stored in the database
as part of the `album.cover_art_path` column; if embedded cover art is
matched, this will default to the path of the media file itself, and if
no cover art is matched at all.
Similarly, the `album.cover_art_id` column will default to a reference
to `media_file.id` if embedded cover art is wanted, but if an external
cover art file is matched, this will instead be set to a reference to
the `album.id` value itself, prefixed with the `al-` constant.
Stored cover art paths are once again resolved and matched against
configuration when covers are requested; that is, any change in
configuration between scanning and requesting cover art may not return
correct data until a re-scan is complete.
Tests will be added in future commits.
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