Core Checkbox

[viridia]

Objective

This is part of the "core widgets" effort: #19236.

Solution

This adds the "core checkbox" widget type.

Testing

Tested using examples core_widgets and core_widgets_observers.

Note to reviewers: I reorganized the code in the examples, so the diffs are large because of code moves.

[ickshonpe]

Is there a reason for querying for Has<InteractedDisabled> instead of using a Without<InteractionDisabled filter, or is it just stylistic?

[ickshonpe]

These constraints don't do anything?

Suggested change

	display: Display::Flex,
	flex_direction: FlexDirection::Row,
	justify_content: JustifyContent::FlexStart,
	align_items: AlignItems::Center,
	align_content: AlignContent::Center,
	align_items: AlignItems::Center,

[ickshonpe]

Suggested change

	Node {
	display: Display::Flex,
	width: Val::Px(16.0),
	height: Val::Px(16.0),
	border: UiRect::all(Val::Px(2.0)),
	..default()
	},
	BorderColor::all(CHECKBOX_OUTLINE), // Border color for the checkbox
	BorderRadius::all(Val::Px(3.0)),
	children![
	// Checkbox inner
	(
	Node {
	display: Display::Flex,
	width: Val::Px(8.0),
	height: Val::Px(8.0),
	position_type: PositionType::Absolute,
	left: Val::Px(2.0),
	top: Val::Px(2.0),
	..default()
	},
	Node {
	width: Val::Px(16.0),
	height: Val::Px(16.0),
	border: UiRect::all(Val::Px(2.0)),
	padding: UiRect::all(Val::Px(2.0)),
	..default()
	},
	BorderColor::all(CHECKBOX_OUTLINE), // Border color for the checkbox
	BorderRadius::all(Val::Px(3.0)),
	children![
	// Checkbox inner
	(
	Node {
	flex_grow: 1.,
	..default()
	},