single global observers root node (#25)

plus auto name observer macro `global_observer`

src/main.rs

@@ -145,6 +145,7 @@ fn main() {
     app.add_plugins(hitpoints::plugin);
     app.add_plugins(cash_heal::plugin);
     app.add_plugins(debug::plugin);
+    app.add_plugins(utils::observers::plugin);
 
     app.init_state::<GameState>();