use object type instead of checking kind

lib/src/participant/participant.dart

@@ -77,13 +77,11 @@ abstract class Participant extends DisposableChangeNotifier
   List<TrackPublication> get subscribedTracks =>
       trackPublications.values.where((e) => e.subscribed).toList();
 
-  List<TrackPublication> get videoTracks => trackPublications.values
-      .where((e) => e.kind == lk_models.TrackType.VIDEO)
-      .toList();
+  // Must be implemented by child class
+  List<TrackPublication> get videoTracks;
 
-  List<TrackPublication> get audioTracks => trackPublications.values
-      .where((e) => e.kind == lk_models.TrackType.AUDIO)
-      .toList();
+  // Must be implemented by child class
+  List<TrackPublication> get audioTracks;
 
   /// for internal use
   /// {@nodoc}