fix: MPRIS Seek functionality on Linux

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.

backend/mpris.go

@@ -59,14 +59,14 @@ func NewMPRISHandler(playerName string, pm *PlaybackManager) *MPRISHandler {
 		}
 	})
 	pm.OnSongChange(func(tr, _ *mediaprovider.Track) {
-		if m.connErr == nil {
-			m.evt.Player.OnTitle()
-		}
 		if tr == nil {
 			m.curTrackPath = ""
 		} else {
 			m.curTrackPath = dbusTrackIDPrefix + encodeTrackId(tr.ID)
 		}
+		if m.connErr == nil {
+			m.evt.Player.OnTitle()
+		}
 	})
 	pm.OnVolumeChange(func(vol int) {
 		if m.connErr == nil {