Change how bend is controlled by mouse

[thomthom]

Change to dragging the reference segment. Using Directional cursor from TT_Lib.

Draw a circular plane at mid point of reference segment, with arrows pointing in each direction - to indicate to the user that this can be interacted with.

[thomthom]

Needs more thought. The sagitta isn't ideal either. Close to 0 or 360 degrees the amount of mouse movements will cause large angle change.

Current implementation uses the reference segment as the circumference for a 360 degree bend. The "bend distance" is then used to get a ratio between it and the circumference. That ratio is used to work out an angle - which yield a liner adjustment of the bend.