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/class-wp-plugins-list-table.php

@@ -742,7 +742,7 @@ class WP_Plugins_List_Table extends WP_List_Table {
 
 					// Details link using API info, if available
 					if ( isset( $plugin_data['slug'] ) && current_user_can( 'install_plugins' ) ) {
-						$plugin_meta[] = sprintf( '<a href="%s" class="thickbox" aria-label="%s" data-title="%s">%s</a>',
+						$plugin_meta[] = sprintf( '<a href="%s" class="thickbox open-plugin-details-modal" aria-label="%s" data-title="%s">%s</a>',
 							esc_url( network_admin_url( 'plugin-install.php?tab=plugin-information&plugin=' . $plugin_data['slug'] .
 								'&TB_iframe=true&width=600&height=550' ) ),
 							esc_attr( sprintf( __( 'More information about %s' ), $plugin_name ) ),