gilrs: handle `Relative` axes
Hi,
Just a WIP MR on relative axes. The first two commits are on their respective MRs. I'll remove them when/if the MRs are merged.
I'd appreciate your comments! Thanks!
Relative axis represent a relative unit of movement. Contrary to
absolute axes, they are a bit simpler (no min/max). We can more commonly
find them on mouse devices (vertical/horizontal).
However, we can also find them on devices that have infinite wheels.
They don't have absolute positioning, but they can report relative
movement in either way.