Accessibility: Improve accessibility for the Plugin details modal.

The plugin details modal can be invoked from several screens. There's now a new
`.open-plugin-details-modal` CSS class to be used in combination with the
`.thickbox` CSS class that adds everything needed for accessibility.

- Adds an ARIA role `dialog` and an `aria-label` attribute to the modal
- Adds a `title` attribute to the iframe inside the modal
- Constrains tabbing within the modal
- Restores focus back in a proper place when closing the modal

Also, improves a bit the native Thickbox implementation: it should probably be
replaced with some more modern tool but at least keyboard focus should be moved
inside the modal.

Fixes #33305.

git-svn-id: https://develop.svn.wordpress.org/trunk@36964 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/dashboard.php

@@ -1245,7 +1245,7 @@ function wp_dashboard_plugins_output( $rss, $args = array() ) {
 
 		$ilink = wp_nonce_url('plugin-install.php?tab=plugin-information&plugin=' . $slug, 'install-plugin_' . $slug) . '&TB_iframe=true&width=600&height=800';
 		echo '<li class="dashboard-news-plugin"><span>' . __( 'Popular Plugin' ) . ':</span> ' . esc_html( $raw_title ) .
-			'&nbsp;<a href="' . $ilink . '" class="thickbox" aria-label="' .
+			'&nbsp;<a href="' . $ilink . '" class="thickbox open-plugin-details-modal" aria-label="' .
 			/* translators: %s: plugin name */
 			esc_attr( sprintf( __( 'Install %s' ), $raw_title ) ) . '">(' . __( 'Install' ) . ')</a></li>';