infobars: Intercept some keys when the info bar is shown to allow interacting with it using the keyboard only.

When the info bar is shown tab/shift+tab and escape are intercepted.
* tab/shift+tab change the focus to the info bar buttons which can then be
  cycled through with more tab presses
* escape closes the info bar (same as clicking cancel)

Both keys aren't needed for the document because it's read-only. Other keys,
such arrow/page up/down, remain to the document for navigating.