This is especially important for combined notifications and
notification lists from different sources; the model list should
be sorted to reflect items by timestamp and unread status.
Note: The dm.List and dm.SortedList now mirror the structures
OO.EmitterList and OO.SortedEmitterList that are awaiting to be
added for oojs in Ib94e4e4a49 and I3fd569691549 respectively.
Once those are available, the dm.List and dm.SortedList can
be removed, and the model can mixin OO.SortedEmitterList instead.
Change-Id: I97e1ecbe5dccc478be527a94f037500f78f74b14
