Add loopback capability to backend

[Laurence-RM]

Requires modified PyAudio fork. (Currently only supports Python 3.7)

Use WASAPI capable device to loopback desktop audio from output device.

See backend-v2 branch

[Laurence-RM]

May not be necessary as stereo mix works. But could be used to enable selection of audio device in settings. Removing from Design Prototype milestone.