* Define it even though previous experiments have shown it to be
unnecessary since it seems like it may be necessary after all
* Add QPDF_DLL_CLASS to QPDFObjectHelper and QPDFDocumentHelper in
case there's some future unknown reason why someone may want to have
them and/or in case it helps with the weird
QPDFNameTreeObjectHelper problem.
This is the beginning of higher-level API support using helper
classes. The goal is to be able to add more helpers without continuing
to pollute QPDF's and QPDFObjectHandle's public interfaces.