[mpris] TrackList interface

[Alex Merry]

I've realised an issue with the tracklist interface: AfterTrack (type 
'o') appears in a couple of places (a method and a signal), indicating 
the track to place something after.  The spec says that "before any 
track" should be indicated with an empty path.

However, this is _not_ a valid path value, and libdbus will throw a 
hissy fit and abort the application if you try to marshall such a value.

So, we need some other value to represent "no track".  Maybe "/", the 
root path?

Alex