Implement an editor to customize the touchscreen controls (#14933)

- The editor is accessible via the pause menu and the settings menu.
- Buttons can be moved via drag & drop.
- Buttons can be added/removed. The grid menu added by #14918 is used to show
  all buttons not included in the layout.
- Custom layouts are responsive and adapt to changed screen size / DPI /
  hud_scaling.
- The layout is saved as JSON in the "touch_layout" setting.

src/script/lua_api/l_mainmenu.h

@@ -69,6 +69,8 @@ private:
 
 	static int l_show_keys_menu(lua_State *L);
 
+	static int l_show_touchscreen_layout(lua_State *L);
+
 	static int l_show_path_select_dialog(lua_State *L);
 
 	static int l_set_topleft_text(lua_State *L);