Undo/Redo with OCAF

Implemented the Undo/Redo mechanism for simple shapes drawn like rectangle.
Studied the new OCAF concept added in the OpenCascade 6.3.0, the TObj model. This implements over the raw OCAF layer a more abstract layer closer to the business layer and more appropriate for an object oriented application. The concept looks very good, would save a lot of work but the lack of samples and explanations would probably lead to a lot of research work to understand it.

Decided to work on the OCAF task as the following:
- finish implementing the Extrude, Cut, Fillet with the standard OCAF layer (make them undoable and storable) as planned for the iteration getting a better understanding of this mechanism,
- meanwhile think at the TObj model and see how this can be used to make the OCAF usage simpler.