PlanarPolygon should ensure that a minimum of three control points are added.

All PlanarFigures should ensure that control points cannot be placed directly over an already existing control point of the same PlanarFigure. In other words, it shall not be possible to create a new control point over the (visible) control point of an existing figure. This limitation shall depend on the soom level: the more you zoom in, the closer to each other you can set the control points as long as there is no visible overlap.