Originally SmUnHorNode::Arrange() was somewhat kludgy. This change
implements a similar manner with SmBinHorNode::Arrange().

Change-Id: Ic18d2e7f70becfabb2c651719926e358a4585526
Reviewed-on: https://gerrit.libreoffice.org/26841Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>