Add toString for TabChangedEvent

lib/events/tab_changed_event.dart

@@ -8,6 +8,11 @@ class TabChangedEvent extends Event {
     this.selectedIndex,
     this.source,
   );
+
+  @override
+  String toString() {
+    return 'TabChangedEvent{selectedIndex: $selectedIndex, source: $source}';
+  }
 }
 
 enum TabChangedEventSource {