message: Use `Sandbox` in `tour` example
diff: use iced::{
  - button, executor, scrollable, slider, text_input, Application, Button,
  - Checkbox, Color, Column, Command, Container, Element, HorizontalAlignment,
  - Image, Length, Radio, Row, Scrollable, Settings, Slider, Space, Text,
  - TextInput,
  + button, scrollable, slider, text_input, Button, Checkbox, Color, Column,
  + Container, Element, HorizontalAlignment, Image, Length, Radio, Row,
  + Sandbox, Scrollable, Settings, Slider, Space, Text, TextInput,
  };
  pub fn main() {
  @@ -19,29 +18,24 @@ pub struct Tour {
  debug: bool,
  }
  -impl Application for Tour {
  - type Executor = executor::Null;
  +impl Sandbox for Tour {
  type Message = Message;
  - type Flags = ();
  - fn new(_flags: ()) -> (Tour, Command<Message>) {
  - (
  + fn new() -> Tour {
  Tour {
  steps: Steps::new(),
  scroll: scrollable::State::new(),
  back_button: button::State::new(),
  next_button: button::State::new(),
  debug: false,
  - },
  - Command::none(),
  - )
  + }
  }
  fn title(&self) -> String {
  format!("{} - Iced", self.steps.title())
  }
  - fn update(&mut self, event: Message) -> Command<Message> {
  + fn update(&mut self, event: Message) {
  match event {
  Message::BackPressed => {
  self.steps.go_back();
  @@ -53,8 +47,6 @@ impl Application for Tour {
  self.steps.update(step_msg, &mut self.debug);
  }
  }
  -
  - Command::none()
  }
  fn view(&mut self) -> Element<Message> {
  