[mediaplayer][rfc] make MPRIS a hard dependency?

[hseg]

The mediaplayer blocklet special-cases support for mpd, cmus, and rhythmbox, making all other players go through MPRIS.
However, these all support MPRISv2, albeit through plugins for some:

• https://github.com/eonpatapon/mpDris2
• cmus/cmus@33df57e
• https://gitlab.gnome.org/GNOME/rhythmbox/-/commit/c51af9eca529e3dda2694483240a6bbdf29a0c01
  Any reason for keeping the old code paths around?
  (Note mpv also requires a plugin to support MPRIS, and we don't special-case support for it)