[mpris] MPRIS v2.1

[Conor Curran]

Hi Johannes,

> 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.
>
>    

To be honest, I don't really use playlists but if this is a valid 
use-case then I don't see why we can't cater for this. Something like 
"Ordering-unspecified", whereby the player determines how the playlists 
should be ordered. Thoughts anyone ?

Conor