Add two-input stereo-output microphone mode

[seanogdelaney]

Summary:

• add a Two-in/Stereo-out client audio mode

• send the second input as an auxiliary mono Mic channel

• use the Vocal instrument icon for the Mic channel

• persist and expose the new audio-channel setting

• Implements Two-in/Stereo-out: simplified support for separate mixer channels for a user's instrument and microphone jamulussoftware/jamulus#3756.

• Implements a narrow subset of Multi Channel Fader on one Jamulus jamulussoftware/jamulus#96.

• Related to Allow mono input device and channel selection for stereo input devices. jamulussoftware/jamulus#2344.

Validation:

• Compiles on Linux
• Runs on Linux
• Two channels appear in mixer as expected, with second suffixed 'Mic', using vocal icon.

Notes:
Running two jamulus clients can achieve this, but is a bit awkward.
This way makes it more accessible (less flexible, but easier).