add constructors to Date/DateTime/Time,
that take the css::util counterparts,
to simplify code converting between these type

Change-Id: I4b03da02c63f65b6ec18cb4d6ecc3859bdef1ff7
Reviewed-on: https://gerrit.libreoffice.org/13855Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Eike Rathke <erack@redhat.com>