Display UID and allow setting by a substring of UID

[nbolender]

The device ID is not consistent when unplugging and re-plugging an external audio source (it is some sort of counter). So this presents a problem when you want to choose a specific device.

macOS exposes a UID string for each audio device. The format of this string is a black box. For example, for built-in devices it is a simple string like BuiltInSpeakerDevice, but for a DisplayPort device it may be something like AppleGFXHDAEngineOutputDP:30001:0:{AD19-43B2-5257260B}. In the second case, the same device may appear as AppleGFXHDAEngineOutputDP:30001:1:{AD19-43B2-5257260B} if it is the second DisplayPort device currently plugged in.

So to support switching inputs by UID, this PR checks against a substring and it is left to the user to find the unique portion of the string. For the above example, running SwitchAudioSource -u AD19-43B2-5257260B always selects the appropriate device.