backend-headless: add a seat and some devices

Clients like gtk depend on the wayland server providing a wl_seat.
This adds one and also some dummy devices while at it.

This makes it possible to run the gtk test suite with the headless
backend.
2 jobs for headless-add-seat in 1 minute and 32 seconds
latest
Status Job ID Name Coverage
  Container Prep
passed #376048
container_prep

00:01:00

 
  Build
passed #376049
build-native-meson

00:00:32