fdo#83619 Retain dropdown part after customization

(cherry picked from commit 99ace739828f9d79a2f9b04ef066b90d77f77bc4)

Conflicts:
	sd/source/ui/controller/slidelayoutcontroller.cxx

Change-Id: I84c33e5b646ba43b9cad1d9ed106e7ead911e5c0
Reviewed-on: https://gerrit.libreoffice.org/11466
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>