Accessibility: Media: Add more headings in the Media Modal.

Headings are the predominant mechanism for screen reader users to find information in a page. They also help all users to better identify the main sections of user interfaces.

- adds three new headings within the media modal 
- improves plural form translation for "item selected" by using `wp.i18n`
- horizontally centers the media modal menu in the responsive view

Props kjellr, karmatosed, melchoyce, afercia.
See #47149.
Fixes #47610.


git-svn-id: https://develop.svn.wordpress.org/trunk@46375 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/media.php

@@ -3931,7 +3931,8 @@ function wp_enqueue_media( $args = array() ) {
 		'addToVideoPlaylistTitle'     => __( 'Add to Video Playlist' ),
 
 		// Headings
-		'attachmentsList'             => __( 'Attachments list' ),
+		'filterAttachments'           => __( 'Filter Media' ),
+		'attachmentsList'             => __( 'Media list' ),
 	);
 
 	/**