MPRIS compatibility for libmpv using mpv-mpris

[BzzzThe18th]

Uses mpv-mpris to allow the embed MPV player to communicate through the session bus for media keys, controls, and displays. This has been a feature request for a very long time, and I feel is a necessary improvement, as most modern players work with MPRIS already.

#15 for more context and information.

Changes

• implemented mpv-mpris compatibility for the libmpv implementation
• added separate flatpak debug/release build scripts and configs
• updated README with mpv-mpris (available on package managers
• added socket=session-bus flatpak override
• added filesystem=~/.config/mpv flatpak override

Notes:

• enabling the config property probably breaks a lot of things that I haven't noticed
• haven't added something to copy mpris.so over to the user scripts folder when not using flatpak
• I am entirely new to the codebase, and don't have much rust experience, so there are probably some issues to review
• adding an entire dependency for a feature a handful of people ask for isn't very reasonable, limits compatibility
• session-bus override is a possible security flaw, but talk/own is not effective