Register callback to listen for input events

[Qard]

Currently Input.h just has a condition for to handle window closing, but there is no programmatic control of input yet. I'm currently leaning toward using lambda functions that receive a struct describing input state.

I'm considering having both onKeyDown and whileKeyDown registration methods, so you can get both book-end events and or continuous-stream events. This will likely be useful for stuff like touch gestures later.