Extend Arlington for PPM (ISO 21812-1:2019)

[petervwyatt]

Extend Arlington for ISO 21812-1:2019 Graphic technology — Print product metadata for PDF files — Part 1: Architecture and core requirements for metadata (a.k.a. PPM).

Includes Extension dictionary with /GTSm, etc.

[petervwyatt]

Model has been prototyped but found an issue with the ISO standard (pdf-association/pdf-issues#459).
Reported this to the PDF Association's PPM LWG and also looking for diverse sample PDFs with PPM to test the model.

PS. The ISO spec error was found due to model validation:

Error: can't reach ArrayOfCIP4_PrintProcess from Trailer or XRefStream (assumed as dictionary)
Error: can't reach CIP4_ProductionIntent from Trailer or XRefStream (assumed as dictionary)

(ArrayOfCIP4_PrintProcess is only referenced from inside CIP4_ProductionIntent so is just a dependency.)