webadmin: Allow different options for different entries

ListModelListBoxCell used to have one set of options for all of the
entries. Now it only sets the options in the last minute, when a
specific entry is rendered, according to the items in the ListModel
backing the specific entry, which enables to have different options
for different entries.

Change-Id: I703ab631ef959fa3ca3ad5bb6dd878634993a3b2
Bug-Url: https://bugzilla.redhat.com/989909
Signed-off-by: Lior Vernia <lvernia@redhat.com>