Incorporate velocity into boid motion.

Why does it behave better when heading (aka acceleration) don't come
into the equation for updaing the xy (aka position)? Isn't the equation
for position given acceleration something like this:
    (x t) = (+ (* 1/2 a (square t))
            (* v_0 t)
            x_0)?
Added max-speed to UI
Code cosmetics.
Added a step size of 100 to all the sliders.
Centered all sliders.
The angle of the origin vector is 0 radians.