Add missing property changes

[V02460]

Currently not all affected MPRIS properties are updated when mopidy’s state changes. This PR adds handling for

• the tracklist transitioning between empty/non-empty
• next/prev/play state on song change

[jodal]

It has been a long time since I looked at this project, but I think these changes make sense.

Can you update the three failing tests to match your changes:

FAILED tests/test_events.py::test_track_playback_started_changes_playback_status_and_metadata
FAILED tests/test_events.py::test_track_playback_ended_changes_playback_status_and_metadata
FAILED tests/test_events.py::test_playback_state_changed_changes_playback_status_and_metadata

Please also add a new test for tracklist_changed.

[V02460]

This version fixes the existing tests and adds new unit and integration tests. I hope you don’t mind the two commits doing minor cleanups and adding Python 3.10 to tox.

[jodal]

Even though the README says Current maintainer: Stein Magnus Jodal, this extension is just as much up for adoption as the other extensions where that is explicitly stated. Quoting Mopidy-MPD:

If you want to be the maintainer of Mopidy-MPD, please:

• Make 2-3 good pull requests improving any part of the project.
• Read and get familiar with all of the project's open issues.
• Send a pull request removing this section and adding yourself as the "Current maintainer" in the "Credits" section below. In the pull request description, please refer to the previous pull requests and state that you've familiarized yourself with the open issues.

As a maintainer, you'll be given push access to the repo and the authority to make releases to PyPI when you see fit.

If you're interested in this, just replace my name with your own when completing the last step here.