Add a method to get the title of the current page as soon as it's available

## Use case

I'd like to show the title of the current page in the AppBar. The title should be shown as soon as it's available, not only when the page has finished loading.

I know about the usual workaround to use `evalJavascript('document.title')` that is recommended in other issues like #30 and #192. But that isn't enough for two reasons:
1. `evalJavaScript` doesn't work on Android SDK versions below 19.
2. `evalJavaScript` must be run after the page has finished loading.

## Proposal

Add a method that returns the title from the WebView. You can use [WebView.getTitle()](https://developer.android.com/reference/android/webkit/WebView.html#getTitle()) on Android and [WKWebView.title](https://developer.apple.com/documentation/webkit/wkwebview/1415015-title) on iOS to get it.

To know when the title is available, Android offers [WebChromeClient.onReceivedTitle()](https://developer.android.com/reference/android/webkit/WebChromeClient#onReceivedTitle(android.webkit.WebView,%20java.lang.String)). This should be forwarded to a new event on `FlutterWebviewPlugin`. I don't know if there is something similar for iOS.

まず、ネイティブの WebView 統合と FlutterWebviewPlugin のイベントフローを読みます。Android では、WebView.getTitle() と WebChromeClient.onReceivedTitle() を比較します。iOS では、WKWebView.title と対応する更新メカニズムを調査します。両方のプラットフォームで現在のタイトルを取得でき、タイトルの利用可能性がプラグインを通じて公開されれば完了です。

索引モデルが issue の本文から書いたものです。