Added functions and fixed bugs

【Contents of fix】
■Fixed a display bug when the custom post “Product” slider image is not set.
■Changed to allow the number of custom posts to be displayed to be set as desired.
■Added the ability to add arbitrary items to the list of custom posts “Products”.
・You can change the order of optional items by dragging them after adding them.
・When a URL is entered in the optional items, a link to the entered content will be created automatically.
・Added display/hide options to the existing “External Page URLs” and “Categories”.

The following files have been modified to reflect this update.
【Corrected files】
・admin/inc/cpt.php
・admin/inc/front_page.php
・admin/js/my_script.js
・functions/cpt_cf.php
・language/default.mo
・language/default.po
・language/ja.mo
・language/ja.po
・functions.php
・single-cpt.php
・style.css