The way the notes model works requires every timeline model to have a
different ID. Conversely no other kind of model actually needs an ID.
Therefore it makes sense to have the TimelineModelAggregator manage the
IDs as every timeline model will sooner or later be associated with an
aggregator.
Change-Id: Ib8b2c88ed883351d4e3e156dd13e1dd113c21808
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>
780d4c6a43