backends/native: Add rt-scheduler experimental key to set RT scheduling

This is similar to a change in kwin:
https://blog.martin-graesslin.com/blog/2017/09/kwinwayland-goes-real-time/

If the experimental features key has "rt-scheduler", make it claim the lowest
of RT scheduler priorities, this will be both educated to other RT processes
and improves responsiveness wrt all other processes.

This can only work if mutter/gnome-shell process receives CAP_SYS_NICE
somehow, e.g.: "setcap CAP_SYS_NICE=+ep `which gnome-shell`"

Closes: gnome-shell#921
!460
4 jobs for !460 with wip/carlosg/rt-scheduler in 5 minutes and 14 seconds
detached
Status Job ID Name Coverage
  Review
passed #296919
check-commit-log

00:00:26

 
  Build
passed #296920
build-mutter

00:02:34

 
  Test
passed #296922
can-build-gnome-shell

00:01:08

passed #296921
test-mutter

00:02:13