Improve the path select GUI (#5852)

- Allow lua to chose whatever directories or files can be selected
- Fix selecting directories
- Rename dialog to `guiPathSelectMenu` from `guiFileSelectMenu`
- Rename lua function for opening the menu from `show_file_open_dialog` to `show_path_select_dialog`
- Remove duplicate code and fix code style.

Related changes
- fix `clang-format` whitelist.
- Regenerate minetest.conf.example

src/script/lua_api/l_mainmenu.h

@@ -86,7 +86,7 @@ class ModApiMainMenu: public ModApiBase
 
 	static int l_show_keys_menu(lua_State *L);
 
-	static int l_show_file_open_dialog(lua_State *L);
+	static int l_show_path_select_dialog(lua_State *L);
 
 	static int l_set_topleft_text(lua_State *L);