Support view hierarchy for window layouts

[timfish]

Problem Statement

The view hierarchy RFC details a way to send the apps window layout with events.

Solution Brainstorm

In Electron we have access to all the app window dimensions and locations which could be sent with events using this attachment type.

[timfish]

So looking at this again... view hierarchy attachments allow you to list the windows. The complication with desktop apps is that there can be multiple displays with windows on each of them.

Should we list the displays as "windows" with the windows as children?