We actually don't have to save the actual data in the
SortedTimelineModel if we keep the data synchronized to the ranges. This
is easy to do by just keeping track of the indices when new ranges are
inserted. Like that we can eliminate the virtual function calls from
AbstractTimelineModelPrivate and simplify the type hierarchy.

Change-Id: Ia7aa02df57380932b689ddfe9a50ff2031198a7d
Reviewed-by: Kai Koehne <kai.koehne@digia.com>