mpris: Work-around some players not handling empty TrackId

Merged Bastien Nocera requested to merge wip/hadess/mpris-trackid into master

It's unclear what to pass to the SetPosition() method when the application doesn't implement the TrackList interface and so doesn't have TrackId metadata.

Export placeholder /org/mpris/MediaPlayer2/TrackList/NoTrack in the meanwhile, which we'll ignore in when SetPosition() is called.


Closes: #538 (closed)

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