OpenSystemsDevelopment / qpdf

This enables RTTI so we can use dynamic_cast on them across the shared
object boundary.

(patrepl and cleanpatch are my own utilities)

patrepl s/PointerHolder/std::shared_ptr/g {include,libqpdf}/qpdf/*.hh
patrepl s/PointerHolder/std::shared_ptr/g libqpdf/*.cc
patrepl s/make_pointer_holder/std::make_shared/g libqpdf/*.cc
patrepl s/make_array_pointer_holder/QUtil::make_shared_array/g libqpdf/*.cc
patrepl s,qpdf/std::shared_ptr,qpdf/PointerHolder, **/*.cc **/*.hh
git restore include/qpdf/PointerHolder.hh
cleanpatch
./format-code

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.

Preparing to refactor some pattern searching code to use these instead
of their own memchr loops. This should simplify the code that replaces
PCRE.

Also add copyright notice to a few public headers that were missing
one.

InputSource, FileInputSource, and BufferInputSource are now top-level
classes instead of privately nested inside QPDF.