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.

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.

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

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

Move all stream-specific methods to new class qpdf::Stream

Move all array-specific methods to new class qpdf::Array.

Move all dictionary specific methods to BaseDictionary. Create new subclass
Dictionary to allow QPDFObjectHandle to access the moved methods.

Replace data member oh with accessors.

Remove redundant overloads from QPDFObjGen::set.