OpenSystemsDevelopment / qpdf

…`warn`, streamline condition handling.

… warnings, and centralize utility methods.

Introduce null checks for `callbacks` in `QPDFObjectHandle` to allow content streams to be parsed without ParserCallbacks. Update `QPDFJob` to pass `nullptr` instead of `DiscardContents` for parsing.

…std::string_view`, improving performance and code clarity.

…ng encapsulation and maintainability.

…ssing to simplify buffer handling

Simplified checks for empty containers and strings across the codebase using the `.empty()` method. This improves code readability and adheres to best practices for checking emptiness.

Converted multiple occurrences of `count()` to `contains()` throughout the codebase where the goal was to check key existence in containers. This improves code readability and aligns with modern C++ practices, particularly with C++20, making the intent more explicit and potentially aiding performance.

Simplified code by removing redundant `this` qualifiers where they are not essential, improving readability and alignment with coding standards. No changes were made to functionality.

Add static parse methods. Make all external access to QPDFParser through
static methods.

Make all non-static methods including constructors private.

- 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.

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.

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.

The new class is only usable as base classes and does not support direct
instantiation or upcasting.

Add operators to convert to bool, QPDFObjectHandle and QPDFObjGen.

Derive QPDFObjectHandle from BaseHandle and make final.

This improves indentation of long strings. This commit also fixes some
trailing whitespace in ChangeLog.

Also, tidy pipeline constructors and make subclasses final where possible.