[draft] Implement the `popover` API.

[SylvainBoilard]

Hello,

This PR adds support for the popover API.

See:

• https://html.spec.whatwg.org/multipage/popover.html ;
• https://developer.mozilla.org/en-US/docs/Web/API/Popover_API .

Please let me know of any issue; in fact, I would appreciate your input regarding a couple things (see below).

Cheers,

[hhugo]

I apologies for the extra long delay reviewing your PR.