single global observers root node (#25)

plus auto name observer macro `global_observer`

src/heads/mod.rs

@@ -4,6 +4,7 @@ use crate::{
     GameState,
     abilities::HeadAbility,
     backpack::{BackbackSwapEvent, Backpack},
+    global_observer,
     hitpoints::Hitpoints,
     player::{Player, head_id_to_str},
     sounds::PlaySound,
@@ -133,8 +134,8 @@ pub fn plugin(app: &mut App) {
         (reload, sync_hp).run_if(in_state(GameState::Playing)),
     );
 
-    app.add_observer(on_select_active_head);
-    app.add_observer(on_swap_backpack);
+    global_observer!(app, on_select_active_head);
+    global_observer!(app, on_swap_backpack);
 }
 
 fn setup(mut commands: Commands, asset_server: Res<AssetServer>) {