Re-introduce QPDFObject.hh as deprecated

* 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.

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