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/macos/Podfile.lock

@@ -1,18 +1,18 @@
 PODS:
   - device_info_plus_macos (0.0.1):
     - FlutterMacOS
-  - flutter_webrtc (0.7.1):
+  - flutter_webrtc (0.9.0):
     - FlutterMacOS
-    - WebRTC-SDK (= 97.4692.06)
+    - WebRTC-SDK (= 97.4692.07)
   - FlutterMacOS (1.0.0)
   - livekit_client (1.0.0):
     - FlutterMacOS
-    - WebRTC-SDK (~> 97.4692)
+    - WebRTC-SDK (~> 97.4692.07)
   - path_provider_macos (0.0.1):
     - FlutterMacOS
   - shared_preferences_macos (0.0.1):
     - FlutterMacOS
-  - WebRTC-SDK (97.4692.06)
+  - WebRTC-SDK (97.4692.07)
 
 DEPENDENCIES:
   - device_info_plus_macos (from `Flutter/ephemeral/.symlinks/plugins/device_info_plus_macos/macos`)
@@ -42,12 +42,12 @@ EXTERNAL SOURCES:
 
 SPEC CHECKSUMS:
   device_info_plus_macos: 1ad388a1ef433505c4038e7dd9605aadd1e2e9c7
-  flutter_webrtc: a019f551482678a0341c79deacbf8d14ab241ceb
+  flutter_webrtc: 708996d57dcf06da1c6a8e820a43112618d62213
   FlutterMacOS: 57701585bf7de1b3fc2bb61f6378d73bbdea8424
-  livekit_client: 7479527f38e9c4a4dc2d3c58f99ab0dd531d24d6
+  livekit_client: 758f6a39a322a64e2081253b89c66d6f5275816f
   path_provider_macos: 3c0c3b4b0d4a76d2bf989a913c2de869c5641a19
   shared_preferences_macos: a64dc611287ed6cbe28fd1297898db1336975727
-  WebRTC-SDK: eca7a6620a3897801207803672b3d036fb40b9d5
+  WebRTC-SDK: e16ed5e9f54d2a983d5f1f479a4b55f20da202fe
 
 PODFILE CHECKSUM: 6eac6b3292e5142cfc23bdeb71848a40ec51c14c