Add a pointer acceleration API

Add a pointer acceleration API

Only exposes one knob - speed, normalized to a [-1, 1] range with 0 being the
neutral "this is what we think is normal" speed. -1 and 1 reflect the
slowest/fastest reasonable speed on this device.

Note: with this API we commit to having any pointer accelerating as a true
gliding scale. We cannot map the [-1,1] range into a discrete set of steps
as we do not communicate to the caller whether a specific value has changed
the acceleration. Without that, a caller may assume that acceleration has
changed even when it is not visible to the user.

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