Add libinput_event_pointer_get_axis_value_discrete() to count wheel clicks

Add libinput_event_pointer_get_axis_value_discrete() to count wheel clicks

The recent normalization of wheel events means we get the angle in degrees but
we don't know how this corresponds to clicks. The M325 has a 20 degree click
angle, most other mice have 15 degrees. So an angle of 60 can be 3 or 4 click
events.

Most clients care more about the click count than the angle on a mouse wheel.
Provide that value when needed.

Adding a discrete value to the axis event leaves the possibility of defining
discrete units for finger/continuous scroll sources in the future. Right now,
these will always reuturn 0.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>