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* Add ability to restrict children, via ``child_types`` list on the model.
The value can be ``["self", "app.Model", "Model", Model]``.
* Added ability to set ``can_be_root`` to enforce using a node as child.
* Add ``validate_move_to()`` method to perform extra checks on moving children.
* Fix support for UUID fields as primary key.
* **Security notice:** fixed missing permission check on moving nodes, all staff members could move nodes.