and simplify - by passing in a std::unique_ptr to addEvent we avoid the
need to have a virtual clone method

Change-Id: Ie425476a3158c7a66e399c2a9f33d2612dab5cbb
Reviewed-on: https://gerrit.libreoffice.org/67962
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>