OpenSystemsDevelopment / qpdf

- Remove unnecessary QPDF_DLLs
- make deleted constructors public
- move some comments to support tooltips
- modernise some constructors and destructors
- change some member shared pointers to unique pointers

Replace data member oh with accessors.

Remove redundant overloads from QPDFObjGen::set.

Also, change test for QPDFObjGen::isIndirect to obj != 0.
Delete comment from commit afd35f9.

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

ABI breaking change

Have classes contain only a single private member of type
PointerHolder<Members>. This makes it safe to change the structure of
the Members class without breaking binary compatibility. Many of the
classes already follow this pattern quite successfully. This brings in
the rest of the class that are part of the public API.

The Apache License version 2.0 is now the primary license for qpdf.
However, users may, at their option, continue to use Artistic version
2.0.

In internal code and examples, replace calls to getObjectID() and
getGeneration() with calls to getObjGen() where possible.