Realistic accuracy

Adds an optional alternative system for aiming and accuracy.

When shooting, some gaussian noise is added to the aim angles. The amount of noise is determined by the accuracy - roughly, the accuracy percentage the game shows is the chance of the shot being within a tile's width of its target once it's travelled 16 tiles' distance.

The result is that partial cover and range have more realistic and intuitive effects than in vanilla. Cover becomes much more effective. Sniping from across the map requires very high accuracy.

Throwing and other parabolic trajectories are left untouched.