Add support for Mouse Wheel binding

Fixes #2679 (closed) by amending OICS and adding Mouse Wheel event handling to mwinput. I'm not willing to wait for OICS' author to update it on his own time so I've done it myself.

For now mouse-wheel handling for keybinds is handled entirely separately from all other uses of the mouse wheel as mwinput relies on wheel delta being multiplied by some magic number of 120 to work, so I figured I'd leave that alone and put this in alongside it.