First person head bobbing
Closes #5043 by adding configurable head-bobbing when in first-person perspective (off by default).
Also includes matching weapon/hand bobbing and turning inertia, landing bounce, and slightly smoothed sneak activation.
Original Post: A somewhat messy initial commit, I will go over it to clean up later where I can. For now this works as a proof of concept for anyone who wants to try it out.