Finalized implementing a Sketch concept like the ones used in other CAD applications. Each 2D shape will hold a reference to a sketch, a sketch holds as info a plane, the 2D shapes tools will not work without having a sketch active.
The sketch will be useful also when integrating the solver, as the solver uses 2D coordinates to find solutions. We'll use the sketch plane to calculate the 2D points from the 3D ones.
Modified the circle tool to work on sketch. It started to work. Some final polishing needed.