OpenSystemsDevelopment / qpdf

Also, tidy pipeline constructors and make subclasses final where possible.

Change the return type to a std::pair<bool, QPDFObjectHandle> in order to
allow a default constructed object handle (which is currently returned to
indicate failure) to become a valid object.

Refactor Pl_QPDFTokenizer

Prepare for treating indirect references differently depending on whether
we are parsing a PDF file (in which case reference to objects not in the
xref table are null even if they are in the object cache) or whether parse
from user code (in which case an indirect reference can refer to a user
created object).

Create an alternative to getJSON to allow an object handle to be written as JSON without the overhead of creating a JSON object.

Using search and replace.

Add new method QPDF::newReserved()

Add new convenience class QPDFObjGen::Guard