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_ADMIN
somehow.

Closes: gnome-shell#921
!460
3 jobs for !460 with wip/carlosg/rt-scheduler in 6 minutes and 7 seconds (queued for 1 second)
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Status Job ID Name Coverage
  Review
passed #229961
check-commit-log

00:02:50

 
  Build
passed #229962
build-mutter

00:01:16

 
  Test
passed #229963
test-mutter

00:02:00