Add ability to stream audio-only to ChromeCast

As of VLC 3.0-rc1 there is ability to stream video+audio to a chromecast via Video>Render>(Scan)>Chromecastdevice

There are cases when separate audio stream would be useful as well:

Need to use external audio system with Chromecast located away from a projector connected to PC.
Chance to use several android devices as wireless sound receivers (there are ways to set up device Android smartphone as Chromecast and then combine them into set via Google Home app)
When VLC is used to play audio only. "Video>Render" is weird way to use streaming it to a Chromecast.

Edited Jan 08, 2018 by Jean-Baptiste Kempf

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