Simplify setup_scene_once_loaded in animated_fox (#8999)

# Objective

The setup code in `animated_fox` uses a `done` boolean to avoid running
the `play` logic repetitively.

It is a common pattern, but it just work with exactly one fox, and
misses an even more common pattern.

When a user modifies the code to try it with several foxes, they are
confused as to why it doesn't work (#8996).

## Solution

The more common pattern is to use `Added<AnimationPlayer>` as a query
filter.

This both reduces complexity and naturally extend the setup code to
handle several foxes, added at any time.

examples/animation/animated_fox.rs

@@ -83,24 +83,20 @@ fn setup(
 // Once the scene is loaded, start the animation
 fn setup_scene_once_loaded(
  animations: Res<Animations>,
- mut player: Query<&mut AnimationPlayer>,
- mut done: Local<bool>,
+ mut players: Query<&mut AnimationPlayer, Added<AnimationPlayer>>,
 ) {
- if !*done {
- if let Ok(mut player) = player.get_single_mut() {
- player.play(animations.0[0].clone_weak()).repeat();
- *done = true;
- }
+ for mut player in &mut players {
+ player.play(animations.0[0].clone_weak()).repeat();
  }
 }
 
 fn keyboard_animation_control(
  keyboard_input: Res<Input<KeyCode>>,
- mut animation_player: Query<&mut AnimationPlayer>,
+ mut animation_players: Query<&mut AnimationPlayer>,
  animations: Res<Animations>,
  mut current_animation: Local<usize>,
 ) {
- if let Ok(mut player) = animation_player.get_single_mut() {
+ for mut player in &mut animation_players {
  if keyboard_input.just_pressed(KeyCode::Space) {
  if player.is_paused() {
  player.resume();