Create a "widgets" module (by moving title scene)

The non-gameplay scenes are really just a bunch of widgets. I'm going to
put them all together and then bundle the functionality with some
exported plugin builders.

src/widgets.rs

@@ -0,0 +1,99 @@
+use crate::GameState;
+
+use bevy::{
+    color::palettes::css::{BLACK, GREEN, LIGHT_BLUE, RED},
+    prelude::*,
+};
+
+pub fn preparation_widget_plugin(app: &mut App) {
+    app.add_systems(OnEnter(GameState::GetReady), spawn_get_ready)
+        .add_systems(OnExit(GameState::GetReady), despawn_get_ready)
+        .add_systems(
+            Update,
+            (animate_get_ready_widget).run_if(in_state(GameState::GetReady)),
+        )
+        .insert_resource(ReadySetGoTimer(Timer::from_seconds(3.0, TimerMode::Once)));
+}
+
+/// Marker component for things on the get-ready indicator
+#[derive(Component)]
+struct OnReadySetGo;
+
+/// Newtype wrapper for `Timer`. Used to count down during the "get ready" phase.
+#[derive(Deref, DerefMut, Resource)]
+struct ReadySetGoTimer(Timer);
+
+/// Marker for the counter text segment
+#[derive(Component)]
+struct CountdownText;
+
+/// Marker for the counter bar segment
+#[derive(Component)]
+struct CountdownBar;
+
+fn spawn_get_ready(mut commands: Commands) {
+    commands.spawn((
+        OnReadySetGo, // marker, so this can be de-spawned properly
+        Node {
+            align_self: AlignSelf::Center,
+            justify_self: JustifySelf::Center,
+            align_items: AlignItems::Center,
+            justify_content: JustifyContent::Center,
+            flex_direction: FlexDirection::Column,
+            width: Val::Percent(30.),
+            height: Val::Percent(30.),
+            ..default()
+        },
+        BackgroundColor(LIGHT_BLUE.into()),
+        children![
+            (Text::new("Get Ready!"), TextColor(BLACK.into())),
+            (
+                CountdownBar,
+                Node {
+                    width: Val::Percent(90.0),
+                    height: Val::Percent(10.),
+                    ..default()
+                },
+                BackgroundColor(GREEN.into()),
+            ),
+            (
+                CountdownText,
+                Text::new("<uninit timer>"),
+                TextColor(RED.into()),
+            )
+        ],
+    ));
+}
+
+// TODO: Replace this with a generic somewhere else in the crate
+// want: `despawn_screen::<OnReadySetGo>>()`
+fn despawn_get_ready(mut commands: Commands, to_despawn: Query<Entity, With<OnReadySetGo>>) {
+    for entity in to_despawn {
+        commands.entity(entity).despawn();
+    }
+}
+
+fn animate_get_ready_widget(
+    mut text_segment: Single<&mut Text, With<CountdownText>>,
+    mut bar_segment: Single<&mut Node, With<CountdownBar>>,
+    time: Res<Time>,
+    mut timer: ResMut<ReadySetGoTimer>,
+    mut game_state: ResMut<NextState<GameState>>,
+) {
+    // Advance the timer, read the remaining time and write it onto the label.
+    timer.tick(time.delta());
+
+    // Add one to the visual value so the countdown starts at 3 and stops at 1.
+    // Otherwise it starts at 2 and disappears after showing 0.
+    // That feels wrong even though it's functionally identical.
+    let tval = timer.0.remaining().as_secs() + 1;
+    **text_segment = format!("{tval}").into();
+
+    // Shrink the progress bar Node
+    bar_segment.width = Val::Percent(100.0 * (1.0 - timer.fraction()));
+
+    // If the timer has expired, change state to playing.
+    if timer.finished() {
+        game_state.set(GameState::Playing);
+    }
+}