[mpris] MPRIS v2.1

[Johannes Sasongko]

Hi Alex,

The Orderings mechanism is a bit confusing, but I'm especially worried
about this part:

> The available orderings. This must include Alphabetical and either Created or Modified at a minimum.

I don't think that's a reasonable minimum; some players don't record
the creation/modification time of their playlists at all.

Also, it's useful to have a custom ordering option for cases where the
player actually allows users to reorder the playlists manually in
their UI.

Johannes