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

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 gstreamer/gst-plugins-bad#786
8 jobs for agg-pad-drained in 64 minutes and 10 seconds (queued for 105 minutes and 15 seconds)
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passed #174612
gst indent

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passed #174611
manifest

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  Build
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build fedora x86_64

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  Test
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check fedora

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passed #174616
build cerbero android universal

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build cerbero cross win32

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passed #174618
build cerbero cross win64

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passed #174615
build cerbero fedora x86_64

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