frontend: Change ListModel items from Iterable to Collection

Due to a legacy the items in ListModel was of type Iterable which is a very
limited type, since all of the multiple entity results from the backend
come as either a List or a Set, it is safe to change the type to be
a Collection and have the extra functionality from this type

Change-Id: I69fcf4a6340aa74e8fa048d2a1f68bfc8eb99b24
Signed-off-by: Tal Nisan <tnisan@redhat.com>