backends: Add mouse accessibility settings

Add support for mouse accessibility settings to set the click assist
values.

!512
3 jobs for master in 6 minutes and 34 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #319605
build-mutter

00:02:13

 
  Test
passed #319607
can-build-gnome-shell

00:01:15

passed #319606
test-mutter

00:04:20