refactor: add GUI instance reference to UI nodes

src/frontend/UiDocument.hpp

@@ -9,6 +9,7 @@
 #include "io/fwd.hpp"
 
 namespace gui {
+    class GUI;
     class UINode;
 }
 
@@ -45,12 +46,13 @@ public:
     scriptenv getEnvironment() const;
 
     static std::unique_ptr<UiDocument> read(
+        gui::GUI&,
         const scriptenv& parent_env,
         const std::string& name,
         const io::path& file,
         const std::string& fileName
     );
     static std::shared_ptr<gui::UINode> readElement(
-        const io::path& file, const std::string& fileName
+        gui::GUI&, const io::path& file, const std::string& fileName
     );
 };