webadmin: Overload ListModel.setItems() with selectedItem

Added an overload that includes an argument specifying the desired
selectedItem after the items are set; previously this was being set to
null, which could cause problems if the ListModel is associated with
an editor that implements HasConstrainedValue (because then
UiCommonEditorVisitor would override the null value and select the
first item in the collection).

Change-Id: Id6de644297cdd5eb965720fc64aea1a5a735ea24
Bug-Url: https://bugzilla.redhat.com/1078215
Signed-off-by: Lior Vernia <lvernia@redhat.com>