single global observers root node (#25)

plus auto name observer macro `global_observer`

src/movables.rs

@@ -1,5 +1,5 @@
 use crate::{
-    GameState,
+    GameState, global_observer,
     tb_entities::{Movable, MoveTarget},
 };
 use bevy::{prelude::*, utils::HashSet};
@@ -20,10 +20,11 @@ pub struct TriggerMovableEvent(pub HashSet<String>);
 pub fn plugin(app: &mut App) {
     app.register_type::<ActiveMovable>();
     app.add_systems(Update, move_active.run_if(in_state(GameState::Playing)));
-    app.add_observer(trigger);
+
+    global_observer!(app, on_movable_event);
 }
 
-fn trigger(
+fn on_movable_event(
     trigger: Trigger<TriggerMovableEvent>,
     mut commands: Commands,
     uninit_movables: Query<