This adds a richer overlay to the arc-by-three-points tool in Pcbnew,
including more guide-lines and a live display of radius and angle.
Also included is ability to go back to the previous step (if setting end
angle, you can go back to setting start point, etc) using Backspace, and
Ctrl snaps the start/end angles to 45 degree multiples.
This adds new classes
* MULTISTEP_GEOM_MANAGER: represents a generic "geometry manager" that
builds up some geometrical construction based on a sequence of points.
Used by:
* ARC_GEOM_MANAGER: handles the logical flow of constructing an
arc by centre-point, set radius, set angle. This moves the logic out
of the Pcbnew DRAWING_TOOL event loop in drawArc().
* ARC_ASSISTANT: graphical overlay to communicate current arc shape to
the user during the drawing process
John Beard