Add more inputs/events for the `Window`

[dfellis]

There's basic mouse position support, but no mouse clicks, no keyboard keypresses, and no gamepad support.

On the UI front, we'll probably want to expose everything that a web browser has access to, eventually, but this ticket is narrowed down to just input capture.

[dfellis]

Mouse clicks and scroll wheels added. What's left is keyboard inputs and game pad support.

I am thinking that the keyboard tracking should be quasi-booleans like the mouse clicks, registered by a more generic function that takes in a string input representation of the key being pressed, though this may be complicated for various modifier keys like Ctrl, Meta, etc. Maybe just those strings are enough for that?

In any case, the quasi-bool being false/0 when the key is up and true/1 when the key is pressed down.

Then gamepad support would be similar-ish, with actual floats for various axes and quasi-bools for button presses, but it would need both a gamepad index and button/axis selection as two different inputs to select (two different functions there, one for axis input and one for button input).

We will also want some probing functions here, one for keyboard layout (just identifiers, up to the developer to know what to do with that) and one to probe how many gamepad are connected, what it identifies itself as and what axes/buttons are available. Ideally an array of gamepad structs.