Add skeleton code for curve tool.  Current function is the same as the point tool.
Add SEGMENT_CURVE as a ViewMode.
Adding graphics to visualise curves.  Currently a single node is added with material 'yellow' and selected material 'red'.
Add scenepicker filters so the active tool can only pick it's own objects.  Start adding connected nodes to use for spline control points (knots).
Getting the curve tool to work laying down elements closing curves with undo and redo.
Finish the curve tool so that it is working for the 2D view, with undo redo and interpolated segment points.