Major cleanup/rewrite og ConfigBuilder
 - @override decorator and override() method removed, all attributes are automatically overridden
 - New what_built() method on ConfigBuilder
 - ConfigBuilder inserted in dom (printed in json dump) for debug
 - More checks in 'build'
 - ConfigBuilder does not accept '@nested_repeatables' decorator, the relevant decorators are those on the parent and child items

Allow assigning default value to attribute in __init__ body and reassigning later
Some cleanup to demo
More tests