transcriberbin: new high-level bin for speech to Closed Caption
This new element puts together some of the elements we've written in recent times (awstranscriber, tttocea608, textwrap, cccombiner) into a convenience high-level element.
The design of the element is AV in -> AV (+ CC metas) out.
The element exposes property to set and unset a "passthrough" mode, during which the transcriber element's state is set to NULL but kept in the bin, in order for the user to be able to set properties on sub elements no matter what the current mode is, using the GstChildProxy interface.
In addition, the element ensures that the latency it reports stays fixed so that playback continues uninterrupted.