instead of allocating small objects on the heap via std::unique_ptr

Change-Id: Iba1d9ad90dc5a31908027336f85046a9de6f5bc4
Reviewed-on: https://gerrit.libreoffice.org/51769Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>