aggregator: Do not forget to reset the flow return when stoping

Setting it to FLUSHING when the element is not started, and to OK
when it starts.
aggregator: Flush sinkpads when stopping

All values are meaningless in that case, so we should make sure that
we clean everything
aggregator: Reset flow_return *after* stopping the srcpad task.

Otherwise it might be set in an already running aggregate function.
aggregator: Store segment when seeked in READY for later use
videoaggregator: No need to reset segment when starting

It could have been set on READY and we should not wipe it
aggregator: Handle seqnum for caps event
aggregator: set future seqnum before propagating the seek event.

Otherwise we could send caps with the old seqnum.
aggregator: Query seeking when a seek failed to see if it was expected

And do not worry if seeking failed on a stream that is not seekable
audiomixer: Port to GstAggregator
audiomixer: Set the sinkpad segments basetime after seeking

Otherwize stream offset and running time comparission will not be
correct, leading to segfaults after seeks
audiomixer: Handle seeking in READY state
audiomixer: If we're EOS, push EOS, no questions asked.