Update Fallback to match specification

[kylegibson]

According to the ECMA specification, the fallback should inherit the scope of it's grandparent, excluding AlternateContent (so AlternateContent may not contain other AlternateContents).

Currently we have Fallback implemented to allow any child node.

Proposed idea for implementing:

• Add a new XmlField type for lazy evaluation
• Allow referencing the field's parent before content initialization. Currently the parent relationship is setup afterwards.