Accessibility: Improve the Media Library inline uploader accessibility.

For better accessibility, expandable panels should be placed immediately after
the control that expands them. This change moves the Media Library inline
uploader up, right after the "Add New" button, also introducing consistency with
the Plugin and Theme uploaders.
Adds a proper ARIA role on the button and an `aria-expanded` attribute to give
better feedback to assistive technologies users about the uploader's expanded state.
Improves the focus handling when closing the uploader, improves the focus style
and color contrast ratio of the uploader "close" button.

Props mantismamita, karmatosed, adamsilverstein, afercia.
Fixes #37188.

Built from https://develop.svn.wordpress.org/trunk@40359


git-svn-id: http://core.svn.wordpress.org/trunk@40266 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/upload.php

@@ -77,7 +77,7 @@ if ( 'grid' === $mode ) {
 
 		<?php
 		if ( current_user_can( 'upload_files' ) ) { ?>
-			<a href="<?php echo admin_url( 'media-new.php' ); ?>" class="page-title-action"><?php echo esc_html_x( 'Add New', 'file' ); ?></a><?php
+			<a href="<?php echo admin_url( 'media-new.php' ); ?>" class="page-title-action aria-button-if-js"><?php echo esc_html_x( 'Add New', 'file' ); ?></a><?php
 		}
 		?>