single global observers root node (#25)

plus auto name observer macro `global_observer`

src/player.rs

@@ -7,6 +7,7 @@ use crate::{
         Controls,
         controller::{CharacterControllerBundle, MovementBundle, PlayerMovement},
     },
+    global_observer,
     heads::HeadChanged,
     hitpoints::Hitpoints,
     loading_assets::GameAssets,
@@ -57,7 +58,7 @@ pub fn plugin(app: &mut App) {
         (rotate_view_keyboard, rotate_view_gamepad).run_if(in_state(GameState::Playing)),
     );
 
-    app.add_observer(update_head);
+    global_observer!(app, on_update_head);
 }
 
 fn spawn(
@@ -242,7 +243,7 @@ fn toggle_animation(
     }
 }
 
-fn update_head(
+fn on_update_head(
     trigger: Trigger<HeadChanged>,
     mut commands: Commands,
     asset_server: Res<AssetServer>,