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.

example/pubspec.lock

@@ -176,7 +176,7 @@ packages:
       name: flutter_webrtc
       url: "https://pub.dartlang.org"
     source: hosted
-    version: "0.8.12"
+    version: "0.9.0"
   google_fonts:
     dependency: "direct main"
     description: