The PDF code in xmlsecurity served two purposes:

- a generic PDF tokenizer
- signature verification

The first purpose is useful to have in VCL, so the PDF export code can
use it as well when it comes to PDF image handling.

This commit just moves most of the PDF code to VCL, it does not touch
the PDF export code yet. With this, also the somewhat odd xmlsecurity
dependency of CppunitTest_vcl_pdfexport can be removed as well.

Change-Id: I6fe8294ed5c4aa4d79f4b2ddef80a4d1c9d566cc
Reviewed-on: https://gerrit.libreoffice.org/35513Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>