videoaggregator: Notify drained status of a videoaggregator pad

Submitted by Seungha Yang

Link to original bug (#797128)

Description

Add GstVideoAggregatorPad::drained property and
GstVideoAggregator::pad-drained signal to help monitoring composing
status. 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.