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

src/engine/Engine.hpp

@@ -38,6 +38,10 @@ namespace devtools {
     class Editor;
 }
 
+namespace scripting {
+    class IProjectScript;
+}
+
 class initialize_error : public std::runtime_error {
 public:
     initialize_error(const std::string& message) : std::runtime_error(message) {}
@@ -71,6 +75,7 @@ class Engine : public util::ObjectsKeeper {
     std::unique_ptr<Input> input;
     std::unique_ptr<gui::GUI> gui;
     std::unique_ptr<devtools::Editor> editor;
+    std::unique_ptr<scripting::IProjectScript> projectScript;
     PostRunnables postRunnables;
     Time time;
     OnWorldOpen levelConsumer;
@@ -178,4 +183,8 @@ public:
     const Project& getProject() {
         return *project;
     }
+
+    scripting::IProjectScript* getProjectScript() {
+        return projectScript.get();
+    }
 };