Proper seat management that is compatible with libseat

[joleuger]

Currently, for wlroots based wayland compositors, libinput and libseat need to be running and libseat needs to be set to noop-backend to assign everything internally to seat0

Environment=WLR_BACKENDS=headless,libinput
Environment=WLR_LIBINPUT_NO_DEVICES=1
Environment=LIBSEAT_BACKEND=noop

Goal of this issue is to implement proper seat management