Type-aware Publication etc (static typing) (#43)

* implemented

* format

* add missing type

lib/src/publication/local_track_publication.dart

@@ -7,11 +7,11 @@ import '../track/local.dart';
 import '../track/track.dart';
 import 'track_publication.dart';
 
-class LocalTrackPublication extends TrackPublication {
+class LocalTrackPublication<T extends LocalTrack> extends TrackPublication {
   final LocalParticipant _participant;
 
   @override
-  covariant LocalTrack? track;
+  covariant T? track;
 
   LocalTrackPublication(
     lk_models.TrackInfo info,

lib/src/publication/remote_track_publication.dart

@@ -31,9 +31,9 @@ class RendererVisibility {
 
 /// Represents a track publication from a RemoteParticipant. Provides methods to
 /// control if we should subscribe to the track, and its quality (for video).
-class RemoteTrackPublication extends TrackPublication {
+class RemoteTrackPublication<T extends RemoteTrack> extends TrackPublication {
   @override
-  covariant RemoteTrack? track;
+  covariant T? track;
 
   final RemoteParticipant _participant;
   bool _enabled = true;