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
tsdemux: Do not scan for a keyframe in non-accurate mode.
tsdemux: Do not scan for keyframe when in push mode.
audiomixer: add up segment start to the next timestamp.