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/import.php

@@ -101,7 +101,7 @@ if ( empty( $importers ) ) {
 			if ( empty($action) ) {
 				if ( is_main_site() ) {
 					$action = '<a href="' . esc_url( network_admin_url( 'plugin-install.php?tab=plugin-information&plugin=' . $plugin_slug .
-										'&from=import&TB_iframe=true&width=600&height=550' ) ) . '" class="thickbox" title="' .
+										'&from=import&TB_iframe=true&width=600&height=550' ) ) . '" class="thickbox open-plugin-details-modal" title="' .
 										esc_attr__('Install importer') . '">' . $data[0] . '</a>';
 				} else {
 					$action = $data[0];