Move ResponsiveWidgets to example folder

This widget has nothing to do with the library and is only used in
the example, so it is moved to the example.

example/lib/universal_ui/universal_ui.dart

@@ -3,9 +3,9 @@ library universal_ui;
 import 'package:flutter/cupertino.dart';
 import 'package:flutter/foundation.dart';
 import 'package:flutter_quill/flutter_quill.dart';
-
 import 'package:universal_html/html.dart' as html;
 
+import '../widgets/responsive_widget.dart';
 import 'fake_ui.dart' if (dart.library.html) 'real_ui.dart' as ui_instance;
 
 class PlatformViewRegistryFix {

lib/flutter_quill.dart

@@ -8,5 +8,4 @@ export 'src/models/quill_delta.dart';
 export 'src/widgets/controller.dart';
 export 'src/widgets/default_styles.dart';
 export 'src/widgets/editor.dart';
-export 'src/widgets/responsive_widget.dart';
 export 'src/widgets/toolbar.dart';

lib/widgets/responsive_widget.dart

@@ -1,3 +0,0 @@
-/// TODO: Remove this file in the next breaking release, because implementation
-/// files should be located in the src folder, https://bit.ly/3fA23Yz.
-export '../../src/widgets/responsive_widget.dart';