Add files for the reactive example

ReactiveExample/LoginView.cs

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+using System.Reactive.Disposables;
+using System.Reactive.Linq;
+using NStack;
+using ReactiveUI;
+using Terminal.Gui;
+
+namespace ReactiveExample {
+	public class LoginView : Window, IViewFor<LoginViewModel> {
+		readonly CompositeDisposable _disposable = new CompositeDisposable();
+		
+		public LoginView (LoginViewModel viewModel) : base("Reactive Extensions Example") {
+			ViewModel = viewModel;
+			this.StackPanel (new Label ("Login Form"))
+				.Append (UsernameLengthLabel ())
+				.Append (UsernameInput ())
+				.Append (PasswordLengthLabel ())
+				.Append (PasswordInput ())
+				.Append (ValidationLabel ())
+				.Append (LoginButton ())
+				.Append (LoginProgressLabel ());
+		}
+		
+		public LoginViewModel ViewModel { get; set; }
+
+		protected override void Dispose (bool disposing) {
+			_disposable.Dispose ();
+			base.Dispose (disposing);
+		}
+
+		TextField UsernameInput () {
+			var usernameInput = new TextField (ViewModel.Username) { Width = 40 };
+			ViewModel
+				.WhenAnyValue (x => x.Username)
+				.BindTo (usernameInput, x => x.Text)
+				.DisposeWith (_disposable);
+			usernameInput
+				.Events ()
+				.TextChanged
+				.Select (old => usernameInput.Text.ToString ())
+				.DistinctUntilChanged ()
+				.BindTo (ViewModel, x => x.Username)
+				.DisposeWith (_disposable);
+			return usernameInput;
+		}
+
+		Label UsernameLengthLabel () {
+			var usernameLengthLabel = new Label { Width = 40 };
+			ViewModel
+				.WhenAnyValue (x => x.UsernameLength)
+				.Select (length => ustring.Make ($"Username ({length} characters)"))
+				.BindTo (usernameLengthLabel, x => x.Text)
+				.DisposeWith (_disposable);
+			return usernameLengthLabel;
+		}
+
+		TextField PasswordInput () {
+			var passwordInput = new TextField (ViewModel.Password) { Width = 40 };
+			ViewModel
+				.WhenAnyValue (x => x.Password)
+				.BindTo (passwordInput, x => x.Text)
+				.DisposeWith (_disposable);
+			passwordInput
+				.Events ()
+				.TextChanged
+				.Select (old => passwordInput.Text.ToString ())
+				.DistinctUntilChanged ()
+				.BindTo (ViewModel, x => x.Password)
+				.DisposeWith (_disposable);
+			return passwordInput;
+		}
+
+		Label PasswordLengthLabel () {
+			var passwordLengthLabel = new Label { Width = 40 };
+			ViewModel
+				.WhenAnyValue (x => x.PasswordLength)
+				.Select (length => ustring.Make ($"Password ({length} characters)"))
+				.BindTo (passwordLengthLabel, x => x.Text)
+				.DisposeWith (_disposable);
+			return passwordLengthLabel;
+		}
+
+		Label ValidationLabel () {
+			var error = ustring.Make("Please, enter user name and password.");
+			var success = ustring.Make("The input is valid!");
+			var validationLabel = new Label(error) { Width = 40 };
+			ViewModel
+				.WhenAnyValue (x => x.IsValid)	
+				.Select (valid => valid ? success : error)
+				.BindTo (validationLabel, x => x.Text)
+				.DisposeWith (_disposable);
+			ViewModel
+				.WhenAnyValue (x => x.IsValid)	
+				.Select (valid => valid ? Colors.Base : Colors.Error)
+				.BindTo (validationLabel, x => x.ColorScheme)
+				.DisposeWith (_disposable);
+			return validationLabel;
+		}
+
+		Label LoginProgressLabel () {
+			var progress = ustring.Make ("Logging in...");
+			var idle = ustring.Make ("Press 'Login' to log in.");
+			var loginProgressLabel = new Label(idle) { Width = 40 };
+			ViewModel
+				.WhenAnyObservable (x => x.Login.IsExecuting)
+				.Select (executing => executing ? progress : idle)
+				.BindTo (loginProgressLabel, x => x.Text)
+				.DisposeWith (_disposable);
+			return loginProgressLabel;
+		}
+
+		Button LoginButton () {
+			var loginButton = new Button ("Login") { Width = 40 };
+			loginButton
+				.Events ()
+				.Clicked
+				.InvokeCommand (ViewModel, x => x.Login)
+				.DisposeWith (_disposable);
+			return loginButton;
+		}
+		
+		object IViewFor.ViewModel {
+			get => ViewModel;
+			set => ViewModel = (LoginViewModel) value;
+		}
+	}
+}