Add support for switching between desktop screenshare and camera inputs

[dob]

I believe the ffmpeg command that captures the input stream specifies the input as "0:0", which represents first video source, first audio source.

ffmpeg -f avfoundation -list_devices true -i ""

Lists the active input devices, with a sample output of:

[AVFoundation input device @ 0x7fb80be0f680] AVFoundation video devices:
[AVFoundation input device @ 0x7fb80be0f680] [0] FaceTime HD Camera
[AVFoundation input device @ 0x7fb80be0f680] [1] Capture screen 0
[AVFoundation input device @ 0x7fb80be0f680] [2] Capture screen 1
[AVFoundation input device @ 0x7fb80be0f680] AVFoundation audio devices:
[AVFoundation input device @ 0x7fb80be0f680] [0] Built-in Microphone

The user should be able to choose amongst the various inputs via a dropdown, and this should update the UI and input device.