Popup interference reborn

This criterion has been significantly revised and accepted by the Low Vision Task Force. New criterion language with a completely rewritten description, and other pertinent sections is in place. Please take a fresh look if you commented previously.

You can view & comment on GitHub here: https://github.com/w3c/wcag21/issues/75

And here's the new SC language for reference:

"Except where popup presentation is controlled by the user agent, all of the following are true when popup content is visible:

*       Trigger: Popup content does not obscure any part of its triggering content.

*       Hover: If a popup is triggered via pointer hover, then the pointer may be moved onto the popup content without loss of visibility.

*       Focus: Popup content remains visible while any of its components, including the trigger, have focus."

Where we define popup as "content which becomes visible only on focus or pointer hover".

Thanks.

PS - This one had been assigned to Wayne, so with his deserved retirement I'm happy to take on its management.

Steve