Support MPRIS D-Bus specification

[cacack]

This was briefly discussed in #84. Fast forward 3 years, maybe things have changed, so I'd like to revive this feature request. Most desktop players I've used emit song information and accept control using MPRIS. This would allow easy integration of pianobar into the DE.

Specification is here.

[PromyLOPh]

It would be nice to have, I agree. Right now it’s just not possible to
integrade DBUS into pianobar’s main loop in a sensible way. Either you
end up rewriting all the interactive stuff in ui_act.c to be
asynchronous (otherwise you can’t answer DBUS requests at the same time)
or you “just fork another thread” and go insane over the multithreading
issues that will arise. Neither of those seems to be worth it, imo.
The third and easiest solution would be a wrapper translating eventcmd
and fifo to DBUS/MPRIS.