add gui.root & add 'app' library to project scripts

doc/en/scripting/builtins/libgui.md

@@ -103,3 +103,9 @@ gui.load_document(
 ```
 
 Loads a UI document with its script, returns the name of the document if successfully loaded.
+
+```lua
+gui.root: Document
+```
+
+Root UI document

doc/ru/scripting/builtins/libgui.md

@@ -100,3 +100,9 @@ gui.load_document(
 ```
 
 Загружает UI документ с его скриптом, возвращает имя документа, если успешно загружен.
+
+```lua
+gui.root: Document
+```
+
+Корневой UI документ

res/project_script.lua

@@ -17,7 +17,7 @@ local function configure_menu()
         end
         return w, h
     end
-    _GUI_ROOT.root:add(
+    gui.root.root:add(
         "<image id='menubg' src='gui/menubg' size-func='DATA.resize_menu_bg' "..
         "z-index='-1' interactive='true'/>", controller)
     menubg = _GUI_ROOT.menubg

res/scripts/stdlib.lua

@@ -189,6 +189,7 @@ end
 _GUI_ROOT = Document.new("core:root")
 _MENU = _GUI_ROOT.menu
 menu = _MENU
+gui.root = _GUI_ROOT
 
 ---  Console library extension ---
 console.cheats = {}

src/logic/scripting/scripting.cpp

@@ -143,6 +143,12 @@ std::unique_ptr<IProjectScript> scripting::load_project_script(
     auto L = lua::get_main_state();
     auto source = io::read_string(script);
     auto env = create_environment(nullptr);
+    lua::pushenv(L, *env);
+    if (lua::getglobal(L, "__vc_app")) {
+        lua::setfield(L, "app");
+    }
+    lua::pop(L);
+
     lua::loadbuffer(L, *env, source, script.name());
     lua::call(L, 0);
     return std::make_unique<LuaProjectScript>(L, std::move(env));