Update the Explainer's proposed API shape to address recent feedback

This PR updates the Explainer's proposed API shape to address recent feedback. Please take a look; thanks!

Here's a recap of the rationale, and unique advantages over previous iterations of this proposal:

Access to the multi-screen change EventTarget is gated by a permission (#30)
Screen information is synchronously available in event handlers functions (#30)
Unclear permission-gating of screen plurality information is resolved (#30)
Naming has been updated for clarity and consistency with existing nomenclature (#29 & #31)
Separate events are fired on screen array and per-screen changes (#12)
The API exposes a more readily apparent currentScreen (#17)

Thanks for your help informing these changes: @pwnall, @jakearchibald, @kenchris, @jsbell, @tomayac

w3c / window-management