Andrea Fercia 9dfb758afc Accessibility: Improve the File Editors interstitial warning.
The warning displayed upon first visit on the File Editors introduced in [41774]
needs to be the only perceivable content in the page for users of assistive
technologies. It looks like a modal but it's not exactly an ARIA dialog, not an
ARIA alert either, and needs some special treatment.

- constrains tabbing within the modal
- uses `wp.a11y.speak()` to make screen readers announce the modal message
- hides all the other page content from assistive technologies using `aria-hidden="true"`

This way, even if users miss the speak message, the warning is actually the only
perceivable content in the page.

Fixes #42110.


git-svn-id: https://develop.svn.wordpress.org/trunk@41876 602fd350-edb4-49c9-b593-d223f7449a82
2017-10-16 17:58:07 +00:00
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