Add the notion of Fold Event.

Some events are sent by a task which generates other events until its accomplishment.
We already have the notion of BeginEvent/EndEvent. But it could be great to have the notion of BeginFoldEvent/EndFoldEvent, in order to show that some events are the "children" of a main task.
It would permit, for example, to hide this child events from the user perspective, because in some case, only the main event is important to record.