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

src/frontend/screens/LevelScreen.cpp

@@ -97,7 +97,6 @@ LevelScreen::LevelScreen(
     animator->addAnimations(assets.getAnimations());
 
     loadDecorations();
-    initializeContent();
 }
 
 LevelScreen::~LevelScreen() {
@@ -112,6 +111,10 @@ LevelScreen::~LevelScreen() {
     engine.getPaths().setCurrentWorldFolder("");
 }
 
+void LevelScreen::onOpen() {
+    initializeContent();
+}
+
 void LevelScreen::initializeContent() {
     auto& content = controller->getLevel()->content;
     for (auto& entry : content.getPacks()) {