OpenSystemsDevelopment / qpdf

#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.

'#' was omitted from the list of chars that need to be escaped.

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

Refactor QPDF_Array

Code tidy re-throwing of exceptions

Tune indentations in QPDFWriter::unparseObject

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.

Add temporary clone of SparseOHArray  to implement non-sparse mode.

Set indent differently for qdf and non-qdf files.
Combine with preceding newline for qdf files.

Warn if catalog type entry is invalid (fixes #810)