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.
Built from https://develop.svn.wordpress.org/trunk@36964


git-svn-id: http://core.svn.wordpress.org/trunk@36932 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-admin/update-core.php

@@ -284,7 +284,7 @@ function list_plugin_updates() {
 		$details_name = sprintf( '<span class="screen-reader-text">%1$s</span>', esc_attr( $plugin_data->Name ) );
 		/* translators: 1: Plugin name 2: Plugin version */
 		$details_text = sprintf( __( 'View %1$s version %2$s details.' ), $details_name, $plugin_data->update->new_version );
-		$details = sprintf( '<a href="%1$s" class="thickbox">%2$s</a>', esc_url( $details_url ), $details_text );
+		$details = sprintf( '<a href="%1$s" class="thickbox open-plugin-details-modal">%2$s</a>', esc_url( $details_url ), $details_text );
 		$checkbox_id =  "checkbox_" . md5( $plugin_data->Name );
 		?>
 		<tr>