Toggleable y-axis mouse support

[sneakernets]

trafficstars

Consider re-adding code that was removed from this commit to be enabled only through a command-line parameter.

I propose -mousey

[RamonUnch]

I agree, this would be nice, I am used to play with vertical mouse movement and I think it would be even better to have a key to toggle vertical mouse movement state like it can be done with crispy-doom's key_togglenovert option.