convert custom list implementation to regular std::vector<OUString>

Change-Id: I6749307d5b72ddc69581b4aa685091383117073d
Reviewed-on: https://gerrit.libreoffice.org/7369
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>