Bust ModelGraph and ModelNode out of the model registry.

+review REVIEW-5264
Allow properties of managed elements to be targeted.

This is a rather naive, inefficient, implementation. It will be later optimized without changing the user behaviour.

+review REVIEW-5264
Add test case for targeting a value property of a managed object.

+review REVIEW-5264
Push the full transitive structure of managed model elements.

 You can now target 'a.b.c' where 'a' is a managed element with an inherent property 'b' of a managed type.

+review REVIEW-5264
Mark off completed story.

+review REVIEW-5264