Organize events (#13)

* move internal events & remove unused events

* `TrackUnpublishedEvent` for local tracks

* update example since we have local track unpublish events

* use flutter_webrtc 0.6.9

lib/src/internal/events.dart

@@ -0,0 +1,37 @@
+// added
+import 'package:flutter_webrtc/flutter_webrtc.dart' as rtc;
+import 'package:meta/meta.dart';
+
+import '../events.dart';
+
+@internal
+abstract class EngineIceStateUpdatedEvent with EngineEvent {
+  final rtc.RTCIceConnectionState iceState;
+  final bool isPrimary;
+  const EngineIceStateUpdatedEvent({
+    required this.iceState,
+    required this.isPrimary,
+  });
+}
+
+@internal
+class EngineSubscriberIceStateUpdatedEvent extends EngineIceStateUpdatedEvent {
+  const EngineSubscriberIceStateUpdatedEvent({
+    required rtc.RTCIceConnectionState state,
+    required bool isPrimary,
+  }) : super(
+          iceState: state,
+          isPrimary: isPrimary,
+        );
+}
+
+@internal
+class EnginePublisherIceStateUpdatedEvent extends EngineIceStateUpdatedEvent {
+  const EnginePublisherIceStateUpdatedEvent({
+    required rtc.RTCIceConnectionState state,
+    required bool isPrimary,
+  }) : super(
+          iceState: state,
+          isPrimary: isPrimary,
+        );
+}