screen sharing for desktop. (#135)

* screen sharing for desktop (WIP).

* use git repo for testing.

* update.

* Add sourceId/frameRate to ScreenShareCaptureOptions.

* Compatible with flutter web.

* Rendering of camera and screenshare video tracks at same time.

* Remote screen sharing is shown first, and fixed sub/unsub button for screenshare widget.

* resolved conflict.

* modify flutter-webrtc to pub version.

* remove unused import.

* chore: Common params for Camera and ScreenShare.

macos/livekit_client.podspec

@@ -16,5 +16,5 @@ Pod::Spec.new do |s|
   s.static_framework    = true
 
   s.dependency 'FlutterMacOS'
-  s.dependency 'WebRTC-SDK', '~> 97.4692'
+  s.dependency 'WebRTC-SDK', '~> 97.4692.07'
 end