Migrate widgets to use `observe` function.

[Freyja-moth]

Bevy 0.17 added the observe function using a bundle effect which makes it easier to add observers to nest hierarchies.

So far the only place that could use this that I've found is

bevy_new_2d/src/theme/widget.rs

Line 110 in 9fd68f6

Children::spawn(SpawnWith(|parent: &mut ChildSpawner| {

Which could now be done as

(
    Name::new("Button"),
    Node::default(),
    children![
        Name::new("Button Inner"),
        Button,
        BackgroundColor(BUTTON_BACKGROUND),
        InteractionPalette {
            none: BUTTON_BACKGROUND,
            hovered: BUTTON_HOVERED_BACKGROUND,
            pressed: BUTTON_PRESSED_BACKGROUND,
        },
        button_bundle,
        observe(action),
        children![(
            Name::new("Button Text"),
            Text(text),
            TextFont::from_font_size(40.0),
            TextColor(BUTTON_TEXT),
            // Don't bubble picking events from the text up to the button.
            Pickable::IGNORE,
        )],
    )]
)

But there may be more that I'm missing

This will require enabling the experimental_bevy_ui_widgets feature on bevy though which may not be desirable.

[benfrankel]

I'm on the fence about this, since we only do this in one place and bsn! with better observer support might come in 0.18. I'd definitely use observe in my own projects in the meantime, though.