In particular when changing row.

1) Teach DbCellControl about "SelectedItems" as known value property.

2) Fix DbListBox::updateFromModel to actually use the SelectedItems
   it reads from the model, as opposed to throwing it away.

Change-Id: I7074c13b3d271bf2362aa059378aa857682a040b
Reviewed-on: https://gerrit.libreoffice.org/17298Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Michael Stahl <mstahl@redhat.com>