OpenSystemsDevelopment / qpdf

Add copy_if_fallback and explain how it differs from copy_if_shared.

* Just removing a header file would cause build errors with no hint as
  to what happened. This way, people get a warning rather than error
  for the life of qpdf 11, and the warning tells them what to do.

* This avoids build surprises resulting from having two versions of
  QPDF headers installed at once. If you were building code out of a
  checkout of qpdf but had an older version installed on your system,
  if your code included <qpdf/QPDFObject.hh>, everything would work,
  but then your code would break without QPDFObject.hh later.

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

Remove any ambiguity around whether old or new value is being
returned.

When there is no context for writing output or error messages, use the
default logger.

This includes the output PDF, streams from --show-object and
attachments from --save-attachment. This also enables --verbose and
--progress to work with saving to stdout.

I decided these were confusing and inconsistent with how JSON works.
They muddle the API rather than improving it.

This reverts commit dc059560e73e0b373a6e54e71b07e3af4b692cb4.

I changed my mind. There's no harm in leaving it deprecated for a
release cycle.

See ChangeLog for rationale for not deprecating it as originally
planned.

Remove the name/number tree object helper constructors that don't take
a QPDF&.