Consider mechanism to inform user that a type of role is a 'popup'

[scottaohara]

Per discussion in #1024, and the general consensus that aria-haspopup has too many legacy attachments to menus and is not necessarily the best way to inform users that a popup of variable type may be exposed.

Rather, creating a new attribute to convey to users that the invoked element is a 'popup' and that navigating away from it will cause it to dismiss.

[chlane]

@scottaohara would the triggering element have aria-haspopup="true" and the actual element have something like "aria-ispopup"?

[scottaohara]

up for discussion