single global observers root node (#25)

plus auto name observer macro `global_observer`

src/backpack/backpack_ui.rs

@@ -1,5 +1,7 @@
 use super::{BackbackSwapEvent, Backpack, UiHeadState};
-use crate::{GameState, heads::HeadsImages, loading_assets::UIAssets, sounds::PlaySound};
+use crate::{
+    GameState, global_observer, heads::HeadsImages, loading_assets::UIAssets, sounds::PlaySound,
+};
 use bevy::prelude::*;
 
 static HEAD_SLOTS: usize = 5;
@@ -51,7 +53,7 @@ pub fn plugin(app: &mut App) {
             .run_if(in_state(GameState::Playing)),
     );
 
-    app.add_observer(swap_head_inputs);
+    global_observer!(app, swap_head_inputs);
 }
 
 fn setup(mut commands: Commands, assets: Res<UIAssets>) {