OpenSystemsDevelopment / qpdf

- Add missing export for operator QPDFObjGen.

- Fix handling of indirect references. The use of the type_code method
  in various switch statements bypassed the special handling of
  ::ot_reference objects. Replace with new method resolved_type_code.

Add tests for ::ot_reference objects.

Enhance error checking of object stream object ids and offsets

The original test file contains multiple entries with id 0 and offset 0.
One entry has been modified such that the id is valid (6).

Object streams with invalid offsets are a source of unreproduceable
oss-fuzz time-outs.

Deprecate QPDFObjectHandle::parse overload and undeprecate isInitialized

Use Tokenizer instead of QPDFTokenizer internally in qpdf

Avoid creating new identical descriptions for each content stream token.

Also remove some shared pointers and use std::string instead of Pl_Buffer
in Pl_QPDFTokenizer.

Modernise constructors, make classes final, etc.

Update release notes and reverse making QPDFObjectHandle final

Move QPDFObject::copy, disconnect, unparse and write_json to BaseHandle

and delay to qpdf 13 to give users notice of the change.

Refactor QPDFTokenizer

Remove remaining QPDFTokenizer private methods.
Remove QPDFTokenizer privileged access to Tokenizer.

Tidy public header files

Add new private Util.hh header to define inline functions and expose as
ordinary functions in QUtil.

- Remove unnecessary QPDF_DLLs
- make deleted constructors public
- move some comments to support tooltips
- modernise some constructors and destructors
- change some member shared pointers to unique pointers

Register crypto

Clean up comments in QPDFObject.hh

Refactor QPDFObject / QPDFObjectHandle