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/about.php

@@ -139,7 +139,7 @@ include( ABSPATH . 'wp-admin/admin-header.php' );
 									'height'    => 550
 								);
 
-								$plugin_link = '<a href="' . esc_url( add_query_arg( $url_args, network_admin_url( 'plugin-install.php' ) ) ) . '" class="thickbox">WordPress REST API</a>';
+								$plugin_link = '<a href="' . esc_url( add_query_arg( $url_args, network_admin_url( 'plugin-install.php' ) ) ) . '" class="thickbox open-plugin-details-modal">WordPress REST API</a>';
 							} else {
 								$plugin_link = '<a href="https://wordpress.org/plugins/rest-api">WordPress REST API</a>';
 							}