Add a 'System default' option to playback device preference

### Problem to be solved

Some DAWs have a preference to use whichever audio output device the computer is currently set to, so that if you change the output device at the system level, the app will automatically start using that device too within the same session.

Audacity is missing this option, so when I switch from laptop work to desk work, I have to change my output device from 'MacBook Air Speakers' to my Scarlett interface (or vice versa) both at the system level _and_ in Audacity preferences.

### Your idea

In Preferences > Audio Settings > Playback > Device, add an option to the top of the list that lets Audacity follow the system audio output device. Potentially do this for 'Recording' device as well, and make it the default selection.

Copy options:

- 'System default' (same as MuseScore Studio)
- 'System setting' (used by Logic)

### Prior art

MuseScore Studio:

Logic: