No volume control possible when streaming to chromecast

When streaming to a chromcast, it's currently impossible to change the output volume of the chromecast using VLC.

If there's no other volume control for the chromecast available, this can be very annoying if the previously set volume was very silent.

Please implement volume control for the chromecast from VLC, it seems like the API to set the volume is pretty straightforward: https://github.com/balloob/pychromecast/blob/2be521e1605eea2af6d67dafa2300507dafd3e3d/pychromecast/socket_client.py#L852

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