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.
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