TinyMCE, improve accessibility:

- Return focus to the editor on pressing Escape while the image toolbar is focused.
- Add a Close button to the Help modal and close it on Escape.
- Override the title on the editor iframe (read by screen reader apps), replace with the Alt+Shift+H shortcut.
- Add focus shortcuts descriptions to the Help modal.
Fixes #27642.
Built from https://develop.svn.wordpress.org/trunk@30757


git-svn-id: http://core.svn.wordpress.org/trunk@30747 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/js/tinymce/plugins/wordpress/plugin.js

@@ -193,8 +193,8 @@ tinymce.PluginManager.add( 'wordpress', function( editor ) {
 			title: 'Keyboard Shortcuts',
 			width: 450,
 			height: 420,
-			inline: 1,
-			classes: 'wp-help'
+			classes: 'wp-help',
+			buttons: { text: 'Close', onclick: 'close' }
 		});
 	});