Merge fullscreen popups & request for feedback

Hi Anssi and SecondScreen groups,

Over the past couple of quarters we have been iterating on fullscreen-popup
<https://github.com/bradtriebwasser/fullscreen-popup> (see also,
Chromestatus <https://chromestatus.com/feature/6002307972464640>),
currently located in my personal GitHub repository. The TAG has also
started an early design review
<https://github.com/w3ctag/design-reviews/issues/840>, asking about group
consensus. I would like to request feedback from Second Screen WG and CG
members, and to move the explainer (PR
<https://github.com/w3c/window-management/pull/140>) and related content
into the main window-management w3c repository
<https://github.com/w3c/window-management>. Please share your feedback
as GitHub
issues <https://github.com/bradtriebwasser/fullscreen-popup/issues> or
directly on the pull request
<https://github.com/w3c/window-management/pull/140>. Your feedback is much
appreciated!

Regards,
Brad