Name-space the scroll event types

To provide a generic naming system of type_direction. That will become more
important once we add new axes as part of the ongoing work to support graphics
tablets.

[edit: and switch to the new defines]

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