[mpris] mpris:trackId in Metadata

[Alex Merry]

I'm writing a compliance-testing application for making sure media 
players implement the MPRIS2 spec correctly, and I've run into a point 
of ambiguity in the spec.

We decided that track ids should be D-Bus object paths, and SetPosition 
and various methods in the TrackList interface accept arguments of type 
'o' for that reason.

However, the description of Metadata in the spec says that mpris:trackId 
should be a string (type 's').  This is how Amarok, for example, has 
implemented it.

So: should the spec stay as it is (with inconsistent types for the track 
id), or should it change to state that the mpris:trackId entry should be 
of D-Bus type 'o'?  Note that the track id is supposed to be an opaque 
value from the POV of MPRIS (although the referenced path might be a 
valid object).

Alex