no need to store small objects like this out of line.

Also
 - add move constructor and move assignment operator
 - drop Assign method since it now has the same
   signature as push_back

Change-Id: I9a8647d3a11f24166a83d399a358a2bce3b2cb79
Reviewed-on: https://gerrit.libreoffice.org/50899Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Eike Rathke <erack@redhat.com>