Implement scroll mouse support
This adds support for the MS Intellimouse & Intellimouse Explorer extensions, as documented by:
https://isdaman.com/alsos/hardware/mouse/ps2interface.htm https://www.scs.stanford.edu/10wi-cs140/pintos/specs/kbd/scancodes-12.html
Wheel state changes & extra buttons are signalled to the OS via PointerReason_WheelChange