timeline: Do not post upstream translated composition update messages
In the case of nested timeline in the toplevel timeline we ended up with CompositionUpdate for seeks sent by our own composition to granchildren composition. This was not causing essential issues if all tracks where containing nested timelines but in cases where one of the tracks only had a nested timelines, then we were waiting forever for a `CompositionUpdateDone`. CompositionUpdate translated into ASYNC_START/ASYNC_DONE should be kept inside the GESTimeline and not travel up (possibly to some parent GESTimeline).
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