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

