PR#194 review

[bsanchezb]

The following changes are applied:

• PdfSigDictWrapper made to contain List<PdfSignatureFields> directly, thus simplifying the logic of interfaces;
• PdfDocumentReader#extractSigDictionaries now return a list of PdfSigDictWrapper's instead of a map between PdfSigDictWrapper and List<PdfSignatureFields> thanks to the change above;
• PdfSigDictWrapper#checkConsistency check reverted to request only PdfSignatureDictionary as an input;
• PdfSigDictWrapper#checkConsistency now performs comparison over the selected signature fields, which means all entries are compared, including the signature dictionary /V;
• Nested validation for some entries are skipped, such as /P and /Parent. Only the top indirect reference is compared.
• OpenPdf implementation has been fixed, to return the actual representation of a signature field, instead of merging it with /AcroForm entries.

All tests now pass with the change. The current PR will be included in DSS 6.6 branch, while some minor adaptations are to be done for DSS 6.5, such as we would evaluate only the essential fields such as /Rectangle and /AP to avoid unexpected regression just before the release.

[bsanchezb]

Edit:

• Moved logic to a new PdfSignatureDictionaryModificationsFinder class;
• Improved PdfObjectModificationsFilter to distinguish between modifications in existing signatures/timestamps.