Hide proxy device when target output device is unavailable

[seungwoochoe]

This PR implements automatic device hiding/showing based on target output availability.

When the proxied audio device is disconnected, the proxy audio device automatically hides itself from the system audio device list. When the target device reconnects, the proxy device reappears. This allows macOS's built-in audio routing logic to take over and automatically switch to the appropriate fallback device like built-in speakers.

[mcg1969]

This looks like a useful but simple improvement. @briankendall any chance we can consider this?

[briankendall]

Sorry for letting this one slide for a while without responding.

This is a good idea, but needs a setting, one that defaults to this feature being off so as not to change the behavior for existing users. One of the major use cases for the proxy device is having a device that doesn't disappear but can still be used to consistently play to the right device, for apps that don't properly handle changes to available audio devices.

[mcg1969]

Very reasonable! Thank you for the response.