Add an input-management layer

[patowen]

Currently, keyboard and mouse input is all handled in an ad-hoc manner in window.rs. Adding an extra layer of abstraction could help simplify the process of adding a new control. We could solve the following problems in one central place (although we don't need to solve them all now to resolve this issue):

• Allowing rebinding of controls
• Adding contextual controls
• Having input presses that trigger an event on the next tick
• Having an axis control for movement
• Detecting conflicts (and allowing the same key to do multiple things if unambiguous enough)
• etc. (This list is not exhaustive.)

Ideally, after this issue is done, if somebody wants to add a new key that does something, they'll just need to add the logic with minimal boilerplate code.

[Ralith]

This is very much needed. I've toyed with the idea of building something reusable inspired by OpenXR/Steam Input, but it's a hard design problem.