protocol: add axis_source.wheel_tilt

Unlike a wheel rotation, a wheel tilt is a discrete-only axis. Wheel rotations
are mapped to degrees in libinput but that that does not apply to wheel tilt
axes where there is no physical equivalent.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Acked-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Yong Bakos <ybakos@humanoriented.com>