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/Gui/ViewProviderSketchGeometryExtension.cpp

@@ -39,13 +39,17 @@ ViewProviderSketchGeometryExtension::ViewProviderSketchGeometryExtension():Repre
 
 }
 
+void ViewProviderSketchGeometryExtension::copyAttributes(Part::GeometryExtension * cpy) const
+{
+    Part::GeometryExtension::copyAttributes(cpy);
+    static_cast<ViewProviderSketchGeometryExtension *>(cpy)->RepresentationFactor = this->RepresentationFactor;
+}
+
 std::unique_ptr<Part::GeometryExtension> ViewProviderSketchGeometryExtension::copy(void) const
 {
     auto cpy = std::make_unique<ViewProviderSketchGeometryExtension>();
 
-    cpy->RepresentationFactor = this->RepresentationFactor;
-
-    cpy->setName(this->getName()); // Base Class
+    copyAttributes(cpy.get());
 
 #if defined (__GNUC__) && (__GNUC__ <=4)
     return std::move(cpy);