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-includes/script-loader.php

@@ -580,9 +580,10 @@ function wp_default_scripts( &$scripts ) {
 			'saved' => __( 'Changes saved.' ),
 		) );
 
-		$scripts->add( 'plugin-install', "/wp-admin/js/plugin-install$suffix.js", array( 'jquery', 'thickbox' ), false, 1 );
+		$scripts->add( 'plugin-install', "/wp-admin/js/plugin-install$suffix.js", array( 'jquery', 'jquery-ui-core', 'thickbox' ), false, 1 );
 		did_action( 'init' ) && $scripts->localize( 'plugin-install', 'plugininstallL10n', array(
-			'plugin_information' => __('Plugin Information:'),
+			'plugin_information' => __( 'Plugin:' ),
+			'plugin_modal_label' => __( 'Plugin details' ),
 			'ays' => __('Are you sure you want to install this plugin?')
 		) );