Add Audio loopback support

[MiroloTech]

As far as I know, it isn't currently possible to get audio output as an input / loopback in Godot. I think it might open a bunch of more potential for background apps like a live wallpaper reacting to live audio, a live music visualizer, or something like a live spectral analyzer, which would be my first intention to use it. It should just be enough, to add a Project Setting under the audio "Input Enabled" setting to allow Output devices to work as input-devices in the AudioServer.input_device setter.