fdo#84408 Toolbar: Insert Field dropdown doesn't contain icons

Change-Id: I3c8db72a84f2a68f1f68a0be1b21e2454707d84a
Reviewed-on: https://gerrit.libreoffice.org/12068
Reviewed-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>
Tested-by: Samuel Mehrbrodt <s.mehrbrodt@gmail.com>