[mpris] MPRIS v2.1

[Alex Merry]

  The Ubuntu folk want a list of (some of) the playlists available from 
a media player for their sound menu.  We ditched the version of this in 
the draft of the spec, because of some scalability problems.

I've implemented an optional interface that MPRIS servers can implement, 
and called the combined thing mpris v2.1 (which is obviously completely 
compatible with v2.0).

The spec is at http://www.randomguy3.me.uk/mpris, and the relevant 
interface is http://www.randomguy3.me.uk/mpris/Playlists.html.

Note that, since D-Bus doesn't have a way of getting a list of 
implemented interfaces (other than parsing the introspection), the only 
way to check for this interface is to try to get a property on it.  We 
could introduce a property on the root interface that gives a list of 
implemented interfaces (like telepathy does), or we could just leave it 
like it is and push the D-Bus guys to provide a way to list interfaces.

Comments?

Alex