Code review comment for lp://qastaging/~azzar1/nux/lp-1228093

Mh, instead of this I think it's better to workaround the issue in unity... By ignoring the horizontal scrolling there (otherwise this would break alt+tab horiz scrolling)...

We can instead define two more mouse states like NUX_EVENT_MOUSEWHEEL_HORIZONTAL and NUX_EVENT_MOUSEWHEEL_VERTICAL. This won't break the api and we can then verify the type of scroll event by just checking the event's mouse_state at unity level.