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