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/lib/widgets/participant_info.dart

@@ -2,16 +2,28 @@ import 'package:eva_icons_flutter/eva_icons_flutter.dart';
 import 'package:flutter/material.dart';
 import 'package:livekit_client/livekit_client.dart';
 
+class ParticipantTrack {
+  ParticipantTrack(
+      {required this.participant,
+      required this.videoTrack,
+      required this.isScreenShare});
+  VideoTrack? videoTrack;
+  Participant participant;
+  final bool isScreenShare;
+}
+
 class ParticipantInfoWidget extends StatelessWidget {
   //
   final String? title;
   final bool audioAvailable;
   final ConnectionQuality connectionQuality;
+  final bool isScreenShare;
 
   const ParticipantInfoWidget({
     this.title,
     this.audioAvailable = true,
     this.connectionQuality = ConnectionQuality.unknown,
+    this.isScreenShare = false,
     Key? key,
   }) : super(key: key);
 
@@ -33,14 +45,23 @@ class ParticipantInfoWidget extends StatelessWidget {
                   overflow: TextOverflow.ellipsis,
                 ),
               ),
-            Padding(
-              padding: const EdgeInsets.only(left: 5),
-              child: Icon(
-                audioAvailable ? EvaIcons.mic : EvaIcons.micOff,
-                color: audioAvailable ? Colors.white : Colors.red,
-                size: 16,
-              ),
-            ),
+            isScreenShare
+                ? const Padding(
+                    padding: EdgeInsets.only(left: 5),
+                    child: Icon(
+                      EvaIcons.monitor,
+                      color: Colors.white,
+                      size: 16,
+                    ),
+                  )
+                : Padding(
+                    padding: const EdgeInsets.only(left: 5),
+                    child: Icon(
+                      audioAvailable ? EvaIcons.mic : EvaIcons.micOff,
+                      color: audioAvailable ? Colors.white : Colors.red,
+                      size: 16,
+                    ),
+                  ),
             if (connectionQuality != ConnectionQuality.unknown)
               Padding(
                 padding: const EdgeInsets.only(left: 5),