Multiple bug fixes

【Contents of the fix】
■Fixed a bug that the background image of the first item does not animate when an image is selected as the first item in the header slider of the top page.
■Fixed a bug that when the next item in the header slider of the top page is Youtube, if you click the dot navigation at the bottom right, you will move to the next item before the playback of Youtube is completed.
■If you click the “Add new item” button with the “Split into 2” slider selected in the header content of the top page, and then switch to the “Full Screen” slider while the new item is displayed, the catchphrase of the newly added item will be changed to “Split into 2” only.
■Corrected the wording of the tabbed article widget (erroneously “pickup articles” → correct “featured articles”)
■Fixed a bug in the tabbed article widget where selecting a pickup article did not display the correct article.
■Fixed a bug that the mega menu (small menu with images) could not be set correctly.
■Changed the Mega Menu settings (to the previous specification where the menu type can be selected in the theme options).
■Fine-tune the load screen
■Deleted unnecessary files.

The following files have been fixed in this update.
【Modified file】
admin/inc/header.php
admin/js/my_script.js
functions.php
functions/slider_ini.php
languages/default.po
languages/ja.mo
languages/ja.po
style.css
widget/tab_post_list.php

【Deleted files】
admin/inc/support.php