PDF subtype support [core, utils]
Add support for PDF subtype property (core, glib backend).
Parse /GTS_PDF(A,E,UA,VT,X)Version from the PDF Information Dictionary into three enums: PDFSubtype, PDFSubtypePart, and PDFSubtypeConformance.
This PDF Format Subtype attribute can indicate if the pdf claims that is compatible with PDF ISO Standards (A, E, UA, VT, X).
Closes #363 (closed)