[MPRIS] Has[Interface] properties on MediaPlayer2 interface

[Alex Merry]

When we added the Playlists interface, we decided not to add a 
HasPlaylists property to the org.mpris.MediaPlayer2 interface (to match 
the HasTrackList property) on the basis that such a property was just as 
optional as the interface itself, and so one could just query a property 
on the Playlists interface to see if it is there.

I think this was a mistake, the reason being that an obvious first thing 
for a client to do upon connecting is to do GetAll on the properties of 
the org.mpris.MediaPlayer2 and org.mpris.MediaPlayer2.Player interfaces. 
  This immediately tells the client whether it's worth looking for the 
TrackList interface, but not whether it's worth looking for the 
Playlists interface.  If the property doesn't exist, the client can 
assume its value to be false.

I'd like to add HasPlaylists to org.mpris.MediaPlayer2, and make a 
policy of always adding such a property for new optional interfaces.

Alex