iTextSharp 5.5.10

* Improvements:
# Tagged PDF and accessibility
- Fixed an issue with tagging of nested lists.
- Improvements in tagging of form XObjects.
- When adding multiple Chunks to a Phrase, there's an optimization of merging them when they have the same properties. But this resulted in their accessibility attributes and roles being overwritten.
# PDF/A
- Corrected the allowed annotation types for PDF/A-1, PDF/A-2 and PDF/A-3.
- Fixed issues with output intents in PDF/A. We incorrectly flagged the generated PDF/A document as invalid when stamping an existing PDF/A document. There was also an issue when merging PDF/A documents.
# Text extraction
- Refactored our text extraction implementation to make it more customizable.

- iText 5.5.8 fixes a problem with digital signatures that was accidentally introduced in version 5.5.7.
- we also improved the verification of OCSP responses.
- There were also problems when signing PDFs that are compliant with the PDF/A-2, PDF/A-3 and ZUGFeRD standard. Those are now fixed.
# Other improvements involve:
- Fonts: there were issues with some Noto fonts, we fixed the range of characters in the ToUnicode table, we provided a fallback mechanism in case of absent OS/2 tables,
- Annotations: fixed some scaling issues and some flattening problems when skew and rotation are involved,
- PdfReader: fixed partial reading when a PDF file is imported as a byte array,
- Tagged PDF: fixed a problem with the structure tree when using page events,
- Page labels: we received code contributions from Nick Park that improve page label extraction.