OpenSystemsDevelopment / qpdf

cmake complains when building with -DZOPFLI-ON as opposed to =ON

Figuring out which classes are part of the public API by using library
symbols is fragile (dependent on specific compiler optimizations) and
unreliable (misses some inline things). Instead, use castxml, a tool
that parses C++ to an abstract syntax tree and generates XML, to get a
reliable accounting of public classes and their sizes.

This requires a special build option.

Use "QPDF" only for the QPDF class itself.

This results in measurable performance improvements to packaged binary
libqpdf distributions. QTC remains available for library users and is
still selectively enabled in CI.

* Change DLL_EXPORT to libqpdf_EXPORTS (internal to the build). The
  new name is cmake's default, is more conventional, and is less
  likely to clash with other symbols.
* Add QPDF_DLL_PRIVATE for non-Windows
* Make logic around when to define QPDF_DLL et al more explicit
* Add detailed comments

This is a massive rewrite of the help text and cli.rst section of the
manual. All command-line flags now have their own help and are
specifically index. qpdf --help is completely redone.

The agogo theme doesn't do very well on narrow screens. The code
blocks were used for shell commands but were using Python syntax
highlighting.