message: Center `tour` example
diff: use iced::{
  button, slider, text::HorizontalAlignment, Align, Button, Checkbox, Color,
  - Column, Element, Image, Length, Radio, Row, Slider, Text, UserInterface,
  + Column, Element, Image, Justify, Length, Radio, Row, Slider, Text,
  + UserInterface,
  };
  pub fn main() {
  @@ -78,11 +79,19 @@ impl UserInterface for Tour {
  .push(controls)
  .into();
  - if self.debug {
  + let element = if self.debug {
  element.explain(Color::BLACK)
  } else {
  element
  - }
  + };
  +
  + Column::new()
  + .width(Length::Fill)
  + .height(Length::Fill)
  + .align_items(Align::Center)
  + .justify_content(Justify::Center)
  + .push(element)
  + .into()
  }
  }
  