Remappable controls

I added the ability to remap controls for mouse/keyboard.

Xbox controls not currently remappable, but they are still in the game (A to shoot, X to focus).

I plan to implement remapping for Xbox controllers later when I eventually improve general controller support, but for now I just wanted to get out something simple for 2 reasons:  first, a friend requested it; second, remapping is very new to me and I need to learn more before I go further with it.