[mpris] Playlist extension

[Conor Curran]

Hi,

 >So I'd be in favor of keeping things simple, as they are in the
 >current proposal

Agreed. I reckon the simple use-case is what we should aim for. Trying 
to accommodate the ins and outs of the different client interpretations 
of what a playlist is might go beyond any current realistic use-case and 
make the whole API overly complex for no real reason.

Below is the same list from Bertrand with one extra ability to able to 
query the current active playlist.

- a playlist is a set of tracks, without any assumption as to how the
set is determined
- you can list the available playlists, with their respective name and icon.
- you can start playing a playlist.
- can query the current active playlist (a playlist which is playing)

Conor