Enhance encryption capabilities

[packdat]

This PR adds support for reading PDF-documents protected by encryption-algorithms that were introduced in PDF version 1.6 and 1.7. (i.e. AES-encryption with a key-length of up to 256 bit) It does not alter the behavior of saving a PDF. While implementing this, i encountered several issues that were also addressed on the way:

• When the length of a stream is incorrectly reported in a PdfDictionary, PDFsharp would throw an exception because the expected endstreamkeyword is not found. In this case, a manual scan for the keyword is now performed; not relying on the reported length.
• Encryption of Unicode-strings was not implemented, now it is
  This in turn fixes #264

I also added test-cases and Test-documents (protected by different encryption-algorithms)

[codecov-commenter]

Codecov Report

Merging #274 (6033e93) into master (2828a90) will increase coverage by 3.94%. The diff coverage is 89.04%.

@@            Coverage Diff             @@
##           master     #274      +/-   ##
==========================================
+ Coverage   26.62%   30.57%   +3.94%     
==========================================
  Files         238      242       +4     
  Lines       23336    23909     +573     
  Branches     2809     2881      +72     
==========================================
+ Hits         6214     7310    +1096     
+ Misses      16629    16062     -567     
- Partials      493      537      +44     

Impacted Files	Coverage Δ
PdfSharpCore/Pdf.Security/PdfSecurityHandler.cs	100.00% <ø> (ø)
PdfSharpCore/Pdf.Security/RC4Encryptor.cs	82.90% <82.90%> (ø)
PdfSharpCore/Pdf.IO/Parser.cs	63.03% <85.71%> (+24.83%)	:arrow_up:
PdfSharpCore/Pdf.Security/EncryptorBase.cs	87.03% <87.03%> (ø)
PdfSharpCore/Pdf.Security/EncryptorFactory.cs	91.66% <91.66%> (ø)
...arpCore/Pdf.Security/PdfStandardSecurityHandler.cs	86.51% <92.00%> (-1.36%)	:arrow_down:
PdfSharpCore/Pdf.Security/AESEncryptor.cs	92.53% <92.53%> (ø)
PdfSharpCore/Pdf/PdfString.cs	40.38% <92.85%> (+8.51%)	:arrow_up:
PdfSharpCore/Pdf.IO/Lexer.cs	79.05% <94.28%> (+12.06%)	:arrow_up:
PdfSharpCore/Pdf.Advanced/PdfObjectStream.cs	80.00% <100.00%> (+80.00%)	:arrow_up:
... and 33 more

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