Part/Sketcher: Refactor Geometry Extensions copy/save/restore AND attachment notification

=========================================================================================

- Long overdue refactor to avoid repetition during save/restore and copy.
- New interface to notify an extension when it is attached. It also enables the extension to gain
a pointer to the geometry container. This is intended to extend the functionality already existing
in Part::Geometry.

src/Mod/Sketcher/App/SolverGeometryExtension.h

@@ -76,6 +76,9 @@ public:
         End = status;
     }
 
+protected:
+    virtual void copyAttributes(Part::GeometryExtension * cpy) const override;
+
 private:
     SolverGeometryExtension(const SolverGeometryExtension&) = default;