Implement axis relative direction

This is useful for the few cases where wheel events are not for
scrolling. For example adjusting the volume in the tray.

In this case having the metadata that the delta is backwards is
important. From a kwin POV it's just proxying the libinput
isNaturalScroll setting to clients.

Tested against "qtbase/examples/widgets/widgets/mousebuttons" with
modified Qt and changing the setting in the UI.

Not mergable until upstream lands.
Relevant link:
https://gitlab.freedesktop.org/whot/wayland/-/merge_requests/1 /
https://gitlab.freedesktop.org/wayland/wayland/-/merge_requests/183

CCBUG: 442789