aggregator: Add signal pad-drained to notify actual drained status of pad

Seungha Yang requested to merge seungha.yang/gstreamer:agg-pad-drained into master

In some complex use cases (such as dynamic add/remove composing images), application might want to know when a stream is completely consumed. The issue is that EOS event cannot respect the consumed status of a stream, because a buffer followed by the EOS might have long duration and therefore the buffer will be alive until running time of the other stream is reached to the buffer's running time.

Closes gst-plugins-bad#786

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