add on_screen_change project script event & fix UIDocument::rebuildIndices & move menu background to project script

src/frontend/screens/MenuScreen.hpp

@@ -13,6 +13,12 @@ public:
     MenuScreen(Engine& engine);
     ~MenuScreen();
 
+    void onOpen() override;
+
     void update(float delta) override;
     void draw(float delta) override;
+
+    const char* getName() const override {
+        return "menu";
+    }
 };