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.

git-svn-id: https://develop.svn.wordpress.org/trunk@30757 602fd350-edb4-49c9-b593-d223f7449a82