peripherals: rename scroll-wheel-emulation-button to scroll-button in trackball, add scroll-button to mouse

peripherals: rename scroll-wheel-emulation-button to scroll-button in trackball, add scroll-button to mouse

Furthermore there is no wheel being emulated, only events being emitted, so I dropped it in the name.

This is coupled with another MR (mutter!3336) on mutter to expose On-Button scrolling from libinput (https://wayland.freedesktop.org/libinput/doc/latest/scrolling.html)