Add null checks around calls to InAppWebView callbacks

This prevents a crash I've been seeing as of v1.1.2 (for the onScrollChanged callback)
and should also prevent any other similar possible crashes.

Also, suppress an unused_field analyzer warning for InAppWebViewController._id.

fix #74, merge #81 #83, fix javascript `...args` parameter of `window.flutter_inappbrowser.callHandler()`

bug fix for android build

Enable setTextZoom function of Android-WebViewSetting

Android dependency 'androidx.core:core' has different version for the compile (1.0.0) and runtime (1.0.1) classpath

 Enable setTextZoom function of Android-WebViewSetting

updated documentation for addJavaScriptHandler method, updated README.md, updated inline webview example

added return value for JavaScriptHandlerCallback, added flutterInAppBrowserPlatformReady event for javascript, breaking change: javaScriptHandlersMap contains only one callback

upgrade to androidX

added support for iOS inline native WebView integrated in the flutter widget tree (#18, #45), fix #54 #38 #26 #32 #33 #29 #15

Fixed bug where passing null to expiresDate failed

[fix] encode resourceURL

Fixed error: Double value cannot be converted to Int because the result would be greater than Int.max in 32-bit devices

fixing a problem in safari webview and adding tabs to examples