Make the microphone selector support aggregate device

[YuantongL]

Now from the input selector on the side panel, you can only select 1 device at a time. Make it so we can select multiple devices and stream the audio from all selected devices.

This could be done using CoreAudio low level APIs to make Aggregate device, but I had found it extremely difficult to work with AVAudioEngine. (Bugs and crashes)

See previous context in Issue #2