and return std::unique_ptr

Not that it helps much, the ownership quickly becomes complex once it
hits the TransferableHelper.

Change-Id: I3c6bd72072e092b71b32e4105fe859fdcea956af
Reviewed-on: https://gerrit.libreoffice.org/59777
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>