Multiple bug fixes / support for WP5.8 widget block editor

【Contents of the fix】】
■Adjusted the display timing of the load screen “Circle”.
■Support for the WordPress 5.8 widget block editor
■Fixed a bug in the top page header slider that caused the slider to stop looping under certain conditions after the YouTube video had finished playing.
■Fixed a bug in which changing the “Carousel Direction” in Theme Options > Top Page > Carousel Settings would also change the order of the carousels.
■When the number of articles is less than or equal to the number of articles registered in the “Top Page” > “Carousel Settings” of the theme options, the specification is changed to not repeat for one article and to repeat for two or more articles.
■Fixed a bug in the display of custom fields in the showroom list in the admin panel.
■Implementation of a function that allows you to set up a carousel from the quick edit screen.

【Modified files】
functions/admin_column.php
functions/head.php
functions/loader_css.php
functions/quick_edit.php
functions/showroom_cf.php
functions/slider_ini.php
languages/ja.mo
languages/ja.po
functions.php
style.css