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.
Fix C++ exception handling when -fvisibility=hidden
Ensure that QPDFExc and QPDFSystemError are marked visible, so that their typeinfo will not be
suppressed when -fvisibility=hidden.
Details:
https://gcc.gnu.org/wiki/Visibility
