OpenSystemsDevelopment / qpdf

* Define POINTERHOLDER_TRANSITION
* Make sure we are getting the intended version of qpdf

Remove paragraph about traversal during destruction since this is still necessary with the
new implementation.

Replace operator== and operator!=, which were testing for the same
underlying object, with isSameObjectAs. This change was motivated by
the fact that pikepdf internally had its own operator== method for
QPDFObjectHandle that did structural comparison. I backed out qpdf's
operator== as a courtesy to pikepdf (in my own testing) but also
because I think people might naturally assume that operator== does a
structural comparison, and isSameObjectAs is clearer in its intent.

Expose functions to the C API to create new loggers and to setLogger
and getLogger for QPDF and QPDFJob.

GNU grep 3.8 started to emit warnings when invoking egrep.
Convert all calls to grep -E.

QPDFValueProxy wasn't a good name for it. We decided the evil of
having the header file be named QPDFObject_private.hh was less than
the evil of having the class be named something other than what it
should have been named.

Preparing to change the class name back to QPDFObject

I decided that it's actually fine to copy a direct object to another
QPDF. Even if we eventually prevent a QPDFObject from having multiple
parents, this could happen if an object is moved.

When a QPDF is destroyed, changing indirect objects to direct nulls
makes them effectively disappear silently when they sneak into other
places. Instead, we should treat this as an error. Adding a destroyed
object type makes this possible.

Also, since it's just there for compatibility, we don't need to add
new object types to it.

The qpdf member was already sufficient. Removing this actually fixed a
few pre-existing issues around detecting foreign ownership and
allowing certain conditions to be warnings rather than exceptions.

Add copy_if_fallback and explain how it differs from copy_if_shared.

It might be worth enabling code scanning at GitHub, which is a
superset of lgtm.

* 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 is in preparation for restoring a QPDFObject.hh to ease the
transition on qpdf_object_type_e.

This commit was created by
* Renaming QPDFObject.cc and QPDFObject.hh
* Replacing QPDFObject\b with QPDFValueProxy (where \b is word
  boundary)
* Running format-code
* Manually resorting files in libqpdf/CMakeLists.txt
* Manually refilling the comment in QPDF.hh near class Resolver