protocol: add wl_pointer's axis relative physical direction

Same as whot/wayland!1 (closed), but only rebased

Implementations:

• Qt: https://codereview.qt-project.org/c/qt/qtwayland/+/454981
• KWin: https://invent.kde.org/plasma/kwin/-/merge_requests/3456
• SDL: https://github.com/libsdl-org/SDL/pull/7106
• wlroots: wlroots/wlroots!4003 (merged)

closes #344 (closed)