Add the ability for iframe to switch url if HTML data changes

[tneotia]

Fixes #337

import 'package:flutter/material.dart';
import 'package:flutter_html/flutter_html.dart';
import 'package:flutter_html/image_render.dart';
import 'package:flutter_html/style.dart';

void main() => runApp(new MyApp());

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      title: 'Flutter Demo',
      theme: new ThemeData(
        primarySwatch: Colors.deepPurple,
      ),
      home: new MyHomePage(title: 'flutter_html Example'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => new _MyHomePageState();
}

String htmlData = """
      <h3>IFrame support:</h3>
              <iframe src="https://bing.com"></iframe>
""";

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar: AppBar(
        title: Text('flutter_html Example'),
        centerTitle: true,
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          setState(() {
            if (htmlData.contains("bing")) {
              htmlData = """<h3>IFrame support:</h3>
              <iframe src="https://google.com"></iframe>
              """;
            } else {
              htmlData = """<h3>IFrame support:</h3>
              <iframe src="https://bing.com"></iframe>
              """;
            }
          });
        },
        child: Icon(Icons.change_history),
      ),
      body: SingleChildScrollView(
        child: Html(
          data: htmlData,
        ),
      ),
    );
  }
}

[erickok]

🤔 What if you have two iframes? Does this work (independently)?

[tneotia]

Hah that's something very important that I overlooked. I'm going to mark this as a draft and come back to it.

[tneotia]

@erickok turns out you only need to assign a key to make it work right, as @The-Redhat commented on #337. Now it should work independently!

[erickok]

Wait but now the whole Html widget is controlled by the single key... What is the impact of this?

[tneotia]

I only assigned the key to the WebView. Each WebView will get a different key when a new IframeContentElement class is created.

[erickok]

Does it have to be a global key? Pretty sure this can be an ObjectKey.

[tneotia]

Sure it could be any sort of Key. Is there an advantage to using one type over another?

[erickok]

Yes, for efficiency.

[tneotia]

@erickok changed the key to UniqueKey, its essentially the same as ObjectKey but doesn't require an Object argument.