Added the aiming arcs (and corrected randomness)

[alin256]

Resolving enhancement #15

Now the aiming arcs have fixed length (constant) and assume perfect steering. They are always on.

About randomness:

To make the process more physical, after this update when randomness is added, the arc can only become wider. I rescaled the slider to %.

• 0% - perfect bias
• 100% - turn between the bias and the straight line.

[ArztKlein]

Wouldn't it be a better idea if the user could enable/disable the extrapolation?

[alin256]

@ArztKlein I added your suggested feature.

[shiffman]

This is great, thank you!!