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.
See https://gitlab.freedesktop.org/mpris/mpris-spec/-/issues/19
Closes: #538 (closed)