All window properties on instance; window vs. window proxy stuff

See nodejs/node#855 for some discussion of how the "proxy global" gets confusing in vm contexts. We more explicitly handle the separation here, trying to ensure that only the proxy global is observable. At the same time, we move all properties of the window to its instance. Per https://heycam.github.io/webidl/#Global this is actually more correct. But it was also pretty hard to get any passing tests without doing this, because proxy globals are pretty wonky, especially when they interact with prototypes of the original global. (My best guess is that this comes down to how V8's proxy global does not really have correct ES proxy semantics.)