Administration: Change all the occurrences of "(opens in a new window)" to "(opens in a new tab)".

Props chetan200891, ianbelanger, afercia.
Fixes #43803.
Built from https://develop.svn.wordpress.org/trunk@43174


git-svn-id: http://core.svn.wordpress.org/trunk@43003 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/widgets/class-wp-widget-custom-html.php

@@ -306,7 +306,7 @@ class WP_Widget_Custom_HTML extends WP_Widget {
 				sprintf(
 					'<span class="screen-reader-text"> %s</span>',
 					/* translators: accessibility text */
-					__( '(opens in a new window)' )
+					__( '(opens in a new tab)' )
 				)
 			);
 			$content .= '</p>';