Resets this Timeline back to the start.
This will set the elapsed time to zero and set all events to be incomplete.
If the Timeline had any events that were set to once that have already
been removed, they will not be present again after calling this method.
If the Timeline isn't currently running (i.e. it's paused or complete) then
calling this method resets those states, the same as calling Timeline.play(true).
This Timeline instance.