webadmin: Added ListModel backed ListBox column

ListModelListBoxColumn (which uses ListModelListBoxCell to render its
elements) represents a CellTable column that presents the items of a
ListModel as a ListBox and allows the user to select an item among
them.

Change-Id: If9c04c5c32e9a36fa959af55399a44ff33d175d8
Signed-off-by: Lior Vernia <lvernia@redhat.com>