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 players: Query<&mut AnimationPlayer, Added<AnimationPlayer>>,
-    mut done: Local<bool>,
 ) {
-    if !*done {
+    for mut player in &mut players {
-        if let Ok(mut player) = player.get_single_mut() {
+        player.play(animations.0[0].clone_weak()).repeat();
-            player.play(animations.0[0].clone_weak()).repeat();
-            *done = true;
-        }
     }
 }
 
 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();