OpenSystemsDevelopment / qpdf

Allow access to private header files when defining data members.

For consistency with similar methods, e.g. replaceObject.

from "qpdf-v2" to "qpdf": [..., ...]

Add additional parameters that will be needed to call QPDF::writeJSON
in partial mode.

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

* Replace --create-from-json=file with --json-input, which causes the
  regular input to be treated as json.
* Eliminate --to-json
* In --json=2, bring back "objects" and eliminate "objectinfo". Stream
  data is never present.
* In --json-output=2, write "qpdf-v2" with "objects" and include
  stream data.

There are no automated tests yet, but committing work so far in
preparation for some refactoring.

Also add stubs for top-level QPDF methods (createFromJSON,
updateFromJSON)