Install spopd as an user systemd service
dspop: Enhance tracks formatting
dpsop: Don't crash when spop is not playing
MPRIS2: update to the latest spec
Fix compilation with recent Glib versions
MPRIS2: start fixing the iface/obj confusion
MPRIS2: partial implementation of the TrackList interface

- doesn't implement the AddTrack and RemoveTrack methods (yet)
- only send a TrackListReplaced signal, no TrackAdded or TrackRemoved
- TrackMetadataChanged is never emitted
- track IDs are not fully spec-conformant: if there are multiple
  occurence of a track within the queue, they will have the same ID

Other than that, it works fine :)