Object Manipulation

> It is now possible to change a HelixArc's parameters using the mouse:
1.  Click and grab an end point
2a. Hold CTRL+SHIFT and move the mouse to change the radius
2b. Hold CTRL and move the mouse to change the angle (change turns or "rotations")
2c. Hold SHIFT and move the mouse to change the pitch ("height" or distance between turns)

> The HelixArc's coloring now indicates its z-position (above or below the (z=0)-plane)