BUG-1091: sever the NormalizedNode/Node relationship

This clarifies the API intent.

Node and NormalizedNode both serve as abstract supertypes which are then
specialized. The artificial Node->NormalizedNode relationship creates
the illusion that NormalizedNode is a semantical sibling of SimpleNode
and CompositeNode -- which is simply not true.

Change-Id: I62f56bc70ee570e719fea7660b7ef36ee59072d6
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-1091: sever the NormalizedNode/Node relationship"