OpenSystemsDevelopment / qpdf

Ensure that links to foreign pages are not broken provided that the page is
subsequently copied.

… object (fixes #1011)

 Change JSONHandler::m to std::unique_ptr and declare Members in implementation file

Apply various Clang-Tidy rules

Avoid unnecessary copying of stream dictionary in QPDF::readObject

Manual edits for cases not handled by search and replace

Using search and replace.

Add new convenience class QPDFObjGen::Guard

#863 uses a single null object for nulls that were previously implicit. In
certain circumstances this shared null object gets destroyed (i.e changed
to a QPDF_Destroyed object) when a QPDF object is destroyed.

Modify the QPDF destructor so that null objects get disconnected from the
dying QPDF object but not destroyed to prevent this from happening.