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

src/graphics/ui/GUI.cpp

@@ -56,6 +56,13 @@ GUI::GUI(Engine& engine)
     store("tooltip", tooltip);
     store("tooltip.label", UINode::find(tooltip, "tooltip.label"));
     container->add(tooltip);
+
+    rootDocument = std::make_unique<UiDocument>(
+        "core:root",
+        uidocscript {},
+        std::dynamic_pointer_cast<gui::UINode>(container),
+        nullptr
+    );
 }
 
 GUI::~GUI() = default;
@@ -74,15 +81,8 @@ std::shared_ptr<Menu> GUI::getMenu() {
 }
 
 void GUI::onAssetsLoad(Assets* assets) {
-    assets->store(
-        std::make_unique<UiDocument>(
-            "core:root",
-            uidocscript {},
-            std::dynamic_pointer_cast<gui::UINode>(container),
-            nullptr
-        ),
-        "core:root"
-    );
+    rootDocument->rebuildIndices();
+    assets->store(rootDocument, "core:root");
 }
 
 void GUI::resetTooltip() {