udev: notify about a removed seat when the last device is removed

udev: notify about a removed seat when the last device is removed

When the last device is removed from a seat, destroy the seat.
This guarantees seat removal events on libinput_suspend() as well.

Note that the seat must be manually unlinked, we can't rely on the unlinking
in libinput_seat_destroy. After the event, the seat's refcount is always > 1,
so the seat will live on until the event is destroyed. We need to make sure
it's not in the list anymore though.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>