Use XInput extension as alternative for relative mouse events

[arcnmx]

evdev grab isn't really that necessary, and would also simplify permissions because screenstub doesn't need to read from /dev/input devices to get relative events.

[arcnmx]

xinput needs fixing in rust-xcb or otherwise choose another X wrapper library or xlib?

x11rb is another new contender but it feels too opinionated to be used in the way I'd like to :<