add form_app (#463)

* add form_app

* revert unrelated changes

* use links in README

* remove unnecessary type annotation

* Remove GlobalKey for sign_in_http sample

* reword comment

* run package:copywriter

* use relative import

* fix quotes

* Add punctuation

* make sure setState() is used in form_widgets sample

* avoid first person pronoun

* store date state in parent widget in form_widgets.dart

experimental/form_app/ios/Runner/AppDelegate.swift

@@ -0,0 +1,13 @@
+import UIKit
+import Flutter
+
+@UIApplicationMain
+@objc class AppDelegate: FlutterAppDelegate {
+  override func application(
+    _ application: UIApplication,
+    didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?
+  ) -> Bool {
+    GeneratedPluginRegistrant.register(with: self)
+    return super.application(application, didFinishLaunchingWithOptions: launchOptions)
+  }
+}