Fix for accesskeys issue #839

Add the method `DomUtils.suppressAccesskeyAction` to prevent the browser focusing elements with `accesskey` set. We need this since the browser currently ignores `preventDefault` for the `accesskey` action, causing the issue in #839.
We return a function to restore the accesskey attribute of the elements. This is bound to the `keyup` listener in `handlerStack`, returning true so that our `onKeyup` handler is still called to prevent the any other `keyup` handlers.

I'd hoped that this would be clear from the comment at the head of this function :(