bezier-curve: Lazy initialization of Endpoint's control points.

Allow for automatic subclassing of Point’s clone method.

Rename Endpoint MIRRORED type to MIRROR.
bezier-curve: Namespace changes to prevent infinite loops.

User interactions use the ‘input’ namespace, ControlPoints moving in
concert with Endpoints are under the ‘relative’ namespace, and sibling
ControlPoints act under the ‘translate’ namespace.

Fix BezierCurve’s linkTo method not considering original curve’s first
control point.