Problem:
The operating system toolbar's component to control Supersonic playback
had its Seek functionality broken on Linux. When playing a song, seeking
for a different position from Supersonic worked as expected, but seeking
from the toolbar did nothing to the current track.
I found that the underlying reason was in `MPRISHandler.SetPosition`, as
the `if m.curTrackPath == trackId` always returned false.
`m.curTrackPath` was the right value, but `trackId` was wrong: On the
first song being played for a tracklist, `trackId` when trying to seek
was set to `/org/mpris/MediaPlayer2/TrackList/NoTrack`. When moving to
the next song, `trackId` then became the Track ID of the previously
played song.
Solution:
Changing the call to `OnTitle()` is enough to fix the issue, as the
MPRIS `Metadata()` generation depends on the value of `m.curTrackPath`.
Because it was being called before changing its value, the received
`trackId` was always off.
Michael Manganiello