Drop the callback interface, replace with libevdev_next_event

Drop the callback interface, replace with libevdev_next_event

Callbacks looked good on paper, but synaptics ran into an issue already that
it just couldn't easily pass around the state needed in the actual event
processing function.

Replace with a new interface that only returns the next event (still reading
more off the fd while doing so).

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