albumartcache: Lookup embedded/local cover art

This behavior is similar in result to what gnome-music used to obtained
from Tracker albumart management, if the albumart cache file does not
exist, it will first lookup embedded, then local images.

If none of those exist, the Grilo lookup paths will kick in as it used
to do.

As we now need an URI to pull embedded albumart from, the "all albums"
query will now return one of the songs from the album.

Bump libmediaart version to contain a gir fix.

https://bugzilla.gnome.org/show_bug.cgi?id=779584