Accessibility: Improve the "Hello Dolly" accessibility.

- adds a visually hidden text to give context to the lyrics
- the text is `Quote from Hello Dolly song, by Jerry Herman:`
- adds a `lang` HTML attribute (when the admin language is not English) to better support assistive technologies 
- adds a `dir` HTML attribute to better support the LTR English lyrics with RTL languages 
- CSS adjustments

Props audrasjb, SergeyBiryukov, danieltj, birgire, karmatosed, desrosj, afercia.
Fixes #43632.


git-svn-id: https://develop.svn.wordpress.org/trunk@44929 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/tests/ajax/UpdatePlugin.php

@@ -158,7 +158,7 @@ class Tests_Ajax_Update_Plugin extends WP_Ajax_UnitTestCase {
 				'plugin'       => 'hello.php',
 				'pluginName'   => 'Hello Dolly',
 				'errorMessage' => 'Plugin update failed.',
-				'oldVersion'   => 'Version 1.7.1',
+				'oldVersion'   => 'Version 1.7.2',
 				'newVersion'   => '',
 				'debug'        => array( 'The plugin is at the latest version.' ),
 			),