Add support for org.mpris.MediaPlayer2.TrackList interface

[freundTech]

Documented here: https://specifications.freedesktop.org/mpris-spec/latest/Track_List_Interface.html

[freundTech]

The spotify web API currently isn't powerful enough to support this feature.
While it's possible to get the "context" of a currently playing song (album or playlist it's from), this doesn't include songs manually added to the queue and doesn't work at all for songs not played from an album or playlist (Radio, playing individual songs, ...)