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/ExternalGeometryExtension.h

@@ -67,10 +67,6 @@ public:
     ExternalGeometryExtension() = default;
     virtual ~ExternalGeometryExtension() override = default;
 
-    // Persistence implementer ---------------------
-    virtual void Save(Base::Writer &/*writer*/) const override;
-    virtual void Restore(Base::XMLReader &/*reader*/) override;
-
     virtual std::unique_ptr<Part::GeometryExtension> copy(void) const override;
 
     virtual PyObject *getPyObject(void) override;
@@ -88,6 +84,11 @@ public:
 
     static bool getFlagsFromName(std::string str, ExternalGeometryExtension::Flag &flag);
 
+protected:
+    virtual void copyAttributes(Part::GeometryExtension * cpy) const override;
+    virtual void restoreAttributes(Base::XMLReader &reader) override;
+    virtual void saveAttributes(Base::Writer &writer) const override;
+
 private:
     ExternalGeometryExtension(const ExternalGeometryExtension&) = default;