Remove properties Children and NamedChildren

[ewoutkramer]

These properties list the properties of an instance, and are replaced by GetElementPairs() and IReadOnlyDictionary. We should remove them or rewrite these as (obsolete) extension methods, so they can be used by people (most notably Forge) to have a migration path. We could keep the original properties in, but throwing NotImplemented and add an Obsolete to them to point to the new extension methods.