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>
Split seats into having a physical and a logical name

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Add a generic libinput_event_get_device() function

After dropping seat evens, all events are now are associated with a device, so
provide a generic accessor function and drop the custom ones.

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

Replaced by specific accessor functions for context, seat and device. This
obsoletes the internal target as well, we just direcly ref the element we need
to instead of temporarily storing it in the target.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Merge the device added/removed events into a single device notify event

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Improve namespacing of event types

Now that the target of an event isn't exposed to the caller anymore, the
namespacing can be associated with a more intuitive one.

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