The Widget-Child extension is designed to be used with Base.build, to create

widget classes which need to be nested inside Widget-Parent based widgets.

For example, it could be used to create MenuItem widgets, which sit inside Menu

widgets, or TreeNode widgets which sit inside Tree widgets.

3.1.1

* No changes

3.1.0

* Add new ROOT_TYPE property to constrain the behavior of the "root" attribute

to instances of a specified type

* Fixes to remove method so that it always returns a reference to the

child removed