Implement the TrackList interface

[jodal]

Currently Mopidy-MPRIS does not implement the MPRIS TrackList interface:

Provides access to a short list of tracks which were recently played or will be played shortly. This is intended to provide context to the currently-playing track, rather than giving complete access to the media player's playlist.

Example use cases are the list of tracks from the same album as the currently playing song or the Rhythmbox play queue.

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Note that the (memory and processing) burden of implementing the TrackList interface and maintaining unique track ids for the playlist can be mitigated by only exposing a subset of the playlist when it is very long (the 20 or so tracks around the currently playing track, for example). This is a recommended practice as the tracklist interface is not designed to enable browsing through a large list of tracks, but rather to provide clients with context about the currently playing track.

Feel free to 👍 this issue if you use an MPRIS client that supports this and are interested in this feature.