single global observers root node (#25)

plus auto name observer macro `global_observer`

src/cutscene.rs

@@ -1,6 +1,7 @@
 use crate::{
     GameState,
     camera::{CameraState, MainCamera},
+    global_observer,
     tb_entities::{CameraTarget, CutsceneCamera, CutsceneCameraMovementEnd},
 };
 use bevy::prelude::*;
@@ -23,7 +24,8 @@ enum CutsceneState {
 pub fn plugin(app: &mut App) {
     app.init_resource::<CutsceneState>();
     app.add_systems(Update, update.run_if(in_state(GameState::Playing)));
-    app.add_observer(on_start_cutscene);
+
+    global_observer!(app, on_start_cutscene);
 }
 
 fn on_start_cutscene(