Investigate using mpv-mpris to monitor media playback status rather than PulseAudio

[mijofa]

Ref: jellyfin/jellyfin-media-player#3

Actually, maybe lok at the 4th comment there and consider that approach.

The main issue I want to solve is when hitting the 'next' button or similar to skip to the next episode,
music & lights start returning to non-playing state while the video stream buffers.
This isn't terrible, but it can get kind of annoying depending how long it takes to buffer.
I could probably 90% solve this with a simple sleep 3 in the PreStop of the media-playback target or something,
but I've never really liked how PulseAudio-systemd-roles.py works and would like to replace it if I can.

I might then be able to use something like hassmpris_agent to improve integration with Home Assistant.
Although it looks I can't really get that in Bullseye any time soon due to the GTK+4 dependency

[mijofa]

This ended up being unnecessary for my purposes as the Home Assistant's Jellyfin integration was able to do what I wanted.