OpenSystemsDevelopment / qpdf

Having it at compile time breaks cross-compilation and isn't really
right anyway.

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

Also, move _qpdf_data and to new header filer _qpdf_error to new header
file qpdf-c_impl.hh

There are no reasons other than historical to use size_t.
On balance, using map is more efficient. Hold shared pointers to
QPDFObjects rather than QPDFObjectHandles for consistencey with
QPDF_Array.

Also, add const overload of QPDFObjectHandle::getObj

Stop using nullptr to represent null objects.
Count null array elements and trigger creation of sparse arrays if null count is
greater than 100.