Add a configuration option for natural scrolling

Add a configuration option for natural scrolling

Natural scrolling is simply inverted scrolling, but I decided to
use the Apple terminology simply because it's easier to google for.

Add the usual quartett of config options for has/set/get/get_default/, as a
boolean option rather than an enum for scroll mode to avoid name collusion
with the (currently in the works) edge scrolling.

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