Improve xref functionality: introduce tref

[kordwarshuis]

Improve xref (external references) functionality.

[kordwarshuis]

Introduction of the “tref”, use the principle of transclusion: https://en.wikipedia.org/wiki/Transclusion
Tref = written out xref at default level + xref

Henk van Cann: “In preparation for setting up a KERI glossary with Spec-Up-T, I'd like to ask you to look at https://en.wikipedia.org/wiki/Transclusion.
Instead of repeating definitions (def) where an xref being written out would suffice: a tref.

The tref retrieves the content of the external definitions in the default level. It prints that into the document (as transclusion works) and links it to the full definition externally.
Thus:

1. No duplicate defs
2. Printable (t)defs on paper and in pdf
3. The refs to such transcluded definitions can become xrefs.”

[kordwarshuis]

Tref functionality integrated in Spec-Up-T.
I leave this open since I think it is not finished yet.