New kind of property--external

I'm using external here in almost the same context as internal properties.

When a reference is made to a file using an internal property, the two files must be (or become) part of the same structure.

An external property always references a well known. When an external reference is destroyed, the well known is checked to see if it is subject to garbage collection. Further, a file is not subject to garbage collection if it is persistent, has a strong reference from another file in the same structure OR has an external reference.

And now we're only considering a file for garbage collection when an external reference is destroyed OR a strong reference from within the same structure is destroyed.

And the property used to identify the well knowns that are in a data set will now be an external property.