This commit improves the extensibility of the various descriptor
classes. The following changes have been made:
- Constructors have been made protected to allow them to be called by
subclasses in a different package
- Model creation has been moved to the relevant snippet class. This
improves the separation of concerns as a descriptor should not be
aware of template-based rendering and the need for a model. The
method is protected to allow it to be overridden by a custom subclass
that uses a custom descriptor subclass.
- Methods that should not be overridden have been made final.
See gh-73
94d5e2d94f