Add event handler methods to the engine structure

engine/src/system.rs

@@ -18,6 +18,7 @@ pub trait System {
     fn initialize(&mut self) {}
     /// Spawn the first entities of the world.
     fn load_world(&mut self, _ctx: &mut SystemContext) {}
+    fn handle_event(&mut self, _ctx: &mut SystemContext) {}
     fn update(&mut self, _ctx: &mut SystemContext) {}
 }
 
@@ -40,4 +41,4 @@ impl SystemManager {
     pub fn remove<S: 'static>(&mut self) {
         self.systems.shift_remove(&TypeId::of::<S>());
     }
-}
+}