move menu pages loader to Lua & add gui.load_document, file.ext(), file.prefix() & add core:gui_util module

src/logic/scripting/scripting_hud.hpp

@@ -1,6 +1,8 @@
 #pragma once
 
 #include <filesystem>
+#include <functional>
+#include <memory>
 #include <string>
 
 #include "typedefs.hpp"
@@ -10,6 +12,11 @@ namespace fs = std::filesystem;
 class Hud;
 class WorldRenderer;
 
+namespace gui {
+    class UINode;
+    using PageLoaderFunc = std::function<std::shared_ptr<UINode>(const std::string&)>;
+}
+
 namespace scripting {
     extern Hud *hud;
     extern WorldRenderer* renderer;
@@ -29,4 +36,6 @@ namespace scripting {
         const fs::path& file,
         const std::string& fileName
     );
+
+    gui::PageLoaderFunc create_page_loader(); 
 }