dbus: multiple fixes related to mpris:trackid URIs

Fix org.mpris.MediaPlayer2.TrackList.TrackAdded including the same mpris:trackid value to both Metadata and AfterTrack fields. Now it correctly sets the right path when the track is added to the start of the playlist.

Use playlist item id instead of its index during format and parsing of mpris:trackid URIs. This will avoid confusion in the event tracks are replaced or moved around.