Expose cover art through `library.get_images()`

As of Mopidy-Beets 4.0.0rc1, the `Album.images` field is no longer set, as the field no longer exists in Mopidy 3.0. We should implement support for the replacement, `LibraryProvider.get_images()`.

See the docs at https://docs.mopidy.com/en/latest/api/backend/#mopidy.backend.LibraryProvider.get_images