smart-mixer: Use the new 'samples-selected' signal to handle queuing in aggregator...

smart-mixer: Use the new 'samples-selected' signal to handle queuing in aggregator pads

Since aggregator introduced queueing in its sinkpads the way we set
properties on the pads is incorrect as it doesn't take it into account.
This fixes the issue by using the newly introduced `samples-selected`
signal in aggregator to set the properties right before the compositing
is done.

Also require the compositor we use to be an aggregator.

And add a validate test for it.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/merge_requests/204>