Renaming "PropertyReferenceValue" to "PropertyReferenceChain," to reflect its non-Value-ness.
The class has no owning component, cannot meet various APIs of Value objects (such as merge,
or addHandler), and has a curious redefinition of getValue() unlike any other Value's.

Watch for a new PropertyReferenceValue, containing a PR chain but acting like a value, soon.

@rev goliver@
@bug W-1867259@
Renaming PropertyReferenceValue, which wasn't much a "value," to PropertyChain.
This is the first step to creating a new PropertyValue that will have handlers
and an owner and address W-1867259, in which nested MapValue subtrees don't
rerender because the (old) PropertyReferenceValue doesn't have handlers to see
the underlying change, nor an owner to mark dirty as a result.

@rev jbuchanan@
@bug W-1867259@
Merging origin/fabbott/master-pending/propertyreferencechain into master