Draft: tablet: add tablet tool scroll support

[louis-goyard]

This aims at implementing #5854.

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:

• sway (this MR)
• libinput: https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/981
• wayland-protocols: https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/288
• wlroots: https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/4598
• mutter (TODO):
• kwin (TODO):
• GTK (TODO):
• QT (TODO):

Implementation explanations:

For now at least, don't treat the tablet tool scroll as a compositor shortcuts for example. As this is a non-discrete scroll, this would be difficult to implement window rotating with it for example.

I wasn't sure about the seatop_down / seatop_default thing, so I basically handed up copying what handle_tablet_tool_tip did. Please tell me if this is wrong.

Also, there is a specific point I'm struggling with about the wlr_pointer_axis_event.pointer element, I will create a comment below to expand on it.

Pipeline is failing because of libinput API changes required to make tablet tool scroll work. I'm not sure about how to handle this from a CI point of view.