Draft: tablet-v2: add axis event to tablet_tool

This aims at implementing button scrolling with a tablet tool (see #729 in libinput)

For tablets that don't have finger features and no wheels, sliders, etc., it can be annoying to scroll on applications that are not specifically designed for tablet use in minds, when you just want to use the tablet instead of the mouse for everyday life.

This is my first attempt at implementing tablet tool scroll, and a way to open the discussion for the tablet tool scroll API. Sorry if things are still in a draft state, I will edit things according to your comments.

This MR doesn't support (for now at least) scroll button lock. I don't know if this is something that I should try to add or not for now.

Related MRs:

• wayland-protocols (this MR)
• libinput: libinput/libinput!981
• wlroots: wlroots/wlroots!4598
• sway: https://github.com/swaywm/sway/pull/8058
• mutter (TODO):
• kwin (TODO):
• GTK (TODO):
• QT (TODO):