refactor: Simplifying universal_ui in the example app

[LuchoTurtle]

In the example app, in order to embed an image into the editor, we need to use platformViewRegistry.registerViewFactory so the image is properly displayed in web applications.

Because platformViewRegistry.registerViewFactory(...) was introduced in flutter/engine#41877, we don't need to use a fake_ui.dart and real_ui.dart to do conditional imports as a workaround for that not compiling on web.

Therefore, we can simply use:

import 'dart:ui_web' as ui_web; // New import for all web-only stuff...

...

ui_web.platformViewRegistry.registerViewFactory(...); // as usual!

This PR makes this simple change and removes consequential dead code 😃 .

[LuchoTurtle]

Although embedding an image on web still doesn't work on the example app (as shown in #976), I was able to verify this to work by returning an image URL in the _onImagePickCallback function inside lib/pages/home_page.dart.

flutter-quill/example/lib/pages/home_page.dart

Lines 329 to 335 in 5dca600

	Future<String> _onImagePickCallback(File file) async {
	// Copies the picked file from temporary cache to applications directory
	final appDocDir = await getApplicationDocumentsDirectory();
	final copiedFile =
	await file.copy('${appDocDir.path}/${basename(file.path)}');
	return copiedFile.path.toString();
	}

Instead of running this, I'm returning a simple URL address, like so (again, just for testing purposes).

  Future<String> _onImagePickCallback(File file) async {
     return "https://i.ytimg.com/vi/CKU8la04tdo/mqdefault.jpg";
  }

[LuchoTurtle]

Closing this as it doesn't properly work on mobile devices :/

Sorry! You can dismiss this if you want to.

[EchoEllet]

We have commented the universal_ui since it's already in the flutter_quill_extensions