Screen Enumeration and Window Placement Update

Hey Second Screen folks,

I have been refining Screen Enumeration
<https://github.com/webscreens/screen-enumeration> and Window Placement
<https://github.com/webscreens/window-placement> explainers and companion
docs to better reflect the scope of our initial proposal.

There are some specific topics where I'd appreciate feedback:

   1. How to handle rejected permission
   <https://github.com/webscreens/screen-enumeration/blob/master/EXPLAINER.md#handling-blocked-permissions>:
reject
   promise & fire exception / fulfill with empty info / fulfill with [
   window.screen ]
   2. API shape for limited info requests
   <https://github.com/webscreens/screen-enumeration/blob/master/EXPLAINER.md#requests-for-limited-information>:
eg.
   getScreens(['multiscreen']) -> { multiscreen: <bool> } to get a single
   bool, perhaps w/o permission
   3. Nuances of window.screen compatibility
   <https://github.com/webscreens/screen-enumeration/blob/master/EXPLAINER.md#compatibility-with-the-existing-windowscreen-object>:
new
   props on window.screen, screen.orientation lock and event handler on other
   screens
   4. Limiting Window Placement V1 scope
   <https://github.com/webscreens/window-placement/blob/master/EXPLAINER.md#current-goals>:
   current goals have been scaled back; lots moved to additional
   explorations
   <https://github.com/webscreens/window-placement/blob/master/additional_explorations.md>

Feel free to share any feedback by filing Github issues, replying here, or
contacting me directly.

Thanks!
Mike