single global observers root node (#25)

plus auto name observer macro `global_observer`

src/aim/marker.rs

@@ -1,4 +1,4 @@
-use crate::{GameState, loading_assets::UIAssets, utils::billboards::Billboard};
+use crate::{GameState, global_observer, loading_assets::UIAssets, utils::billboards::Billboard};
 use bevy::prelude::*;
 use bevy_sprite3d::{Sprite3dBuilder, Sprite3dParams};
 use ops::sin;
@@ -15,7 +15,7 @@ pub enum MarkerEvent {
 
 pub fn plugin(app: &mut App) {
     app.add_systems(Update, move_marker.run_if(in_state(GameState::Playing)));
-    app.add_observer(marker_event);
+    global_observer!(app, marker_event);
 }
 
 fn move_marker(mut query: Query<&mut Transform, With<TargetMarker>>, time: Res<Time>) {