albumart: make a queue of albums being resolved
query: Remove unused queries

Remove count queries for albums and artists, which were used for
Load More buttons that are now gone.
query: Filter out songs that are videos or playlists

Some nmm:MusicPiece are also a nmm:Video, and all nmm:Playlist are also
nmm:MusicPiece objects. We should filter them out because videos don't
have artist and other important fields in grilo, and playlists should
be shown in the Playlists view instead.

https://bugzilla.gnome.org/show_bug.cgi?id=726459
https://bugzilla.gnome.org/show_bug.cgi?id=728255