make line spacing icons point to themes icons

Change-Id: Id93fa68fe28359905f1dd55122fb3b4f48e9710e
Reviewed-on: https://gerrit.libreoffice.org/12274
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>