OpenSystemsDevelopment / qpdf

On destruction of the QPDF object replace all indirect object references
with direct nulls.

Remove all existing code to release resolved references.

Fixes performance issue due to interaction of resetting QPDFValue::qpdf and
og members and prior code.

Now that QPDFObject.hh is out of the public API, we don't really need
QPDFObjAccessor. Also, the function didn't have to be virtual anymore.

This required moving some newly inlined functions back to the cc file,
but that seems to have had no measurable performance impact.

Modify QPDFParser::parse to call QPDF::getObject instead.

Also change QPDF::replaceObject and QPDF::swapObjects such that the
QPDFObject assigned to an og in the obj_cache is never replaced; only
QPDFObject::value is updated.

Centralise casting of QPDFObjects and reduce repeated dereferencing.

Part of #729

Part of #729

Part of #729

Also, inline method and add optional parameter error_msg.

Code tidy: remove redundant calls to smart_ptrs get() method

Also, make QPDFObjectHandle::isIndirect const.

Code tidy : replace 0 with nullptr or true

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

Use 'QPDFObjGen const&' instead of 'int, int' in signature.

Change method signatures to use QPDFObjGen.
Replace QPDF_Stream::objid and generation with QPDF_Stream::og.

Change method signatures to use QPDFObjGen.
Use QPDFObjGen methods where possible.
Remove redundant QPDF::objGenToIndirect.

Change method signature to use QPDFObjGen.

Also, revert inlining of unparse and operator << from commit 4c6640c in
order to avoid exposing QUtil.

Move responsibility for creating shared pointers to objects and cloning from QPDFObjectHandle to QPDFObject.

Also, use methods to access objid and generation.