Implement keyboard camera control

[snshn]

Currently doesn't have support for detecting prolonged key presses, but should be rather easy to add.

Also not sure if currently yaw is responsible for what left/right buttons are expected to control, it seems to be spinning the model rather than "going around it" horizontally.

[mkeeter]

I'd like to hold this until keydown works, since it's unintuitive at the moment.

I may poke at this myself, but no promises – feel free to investigate further if you've got time.