List available Capture devices

[kokorin]

It's currently not possible to get a list of all capture devices available.

There are several ways:

1. Add special abstract ListDevicesInput (like CaptureInput).
   Different output format mat be a problem. It would be required to pass output parser to FFmpeg and to include device list to FFmpegResult.

2. Add separate root class FFmpegDevice which will execute ffmpeg with corresponding options and parse output.

3. Interact directly with ProcessHandler without any options or configuration.

https://trac.ffmpeg.org/wiki/Capture/Desktop
https://ffmpeg.org/ffmpeg-devices.html#Options-7