if ctor should be private or protected explicitly default them.
boost::optional has copy ctors, so use them.

Change-Id: If1855626b297e739afef0dc5ad57958f7ad199bc
Reviewed-on: https://gerrit.libreoffice.org/42363Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>