Drop seat events

Drop seat events

seats are more a compositor concept than a concept of the input library. All
devices in a libinput context are associated with the seat given on creation
of the seat (maps to ID_SEAT in udev for the udev backend).

A logical seat may be assigned to a device (e.g. WL_SEAT) but this does not
necessarily map to the creation of the seat in the compositor.
Drop the seat events but keep seat objects around so that the caller can still
identify which seat a device belongs to.

If the libinput_seat_unref() in the udev backend destroys the seat, the device
list of that seat is invalid and we'd be accessing already freed bytes. To
avoid this, ref the seat before the device removal loop, then unref it once
we're done - that unref then may trigger the actual removal of the seat.

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