aggregator: expose sample selection API
See gst-plugins-base#771 (closed) for context.
This exposes new API that subclasses must call from their aggregate() implementation to signal that they have selected the next samples they will aggregate: gst_aggregator_selected_samples()
GstAggregator will emit a new signal there, samples-selected,
handlers can then look up samples per pad with the newly-added
gst_aggregator_pad_peek_next_sample.
In addition, a new FIXME is logged when subclasses haven't actually
called selected_samples from their aggregate() implementation.