Scope ModelContext to Document, and handle detached documents

[domfarolino]

This PR does two things:

• Scopes the ModelContext object to Document instead of Navigator, and explains why.
• Makes the modelContext attribute return null when the document is detached (by checking when the Window's browsing context is null). This matches the behavior of the contentWindow and contentDocument getters—return nothing, even if those objects are allocated and kept alive by other references, in the detached document case.

Note that this PR does not handle the "document not fully active" case, which covers detached documents and documents in the bf-cache. Non-fully active documents probably shouldn't impact the modelContext getter—it seems weird for getters like that to return different values based on whether the document is in the bf-cache—but rather need special behavior in the registerTool() method, and the coming getTools() and executeTool() methods.

This PR mostly addresses #173, with the exception of considering moving the API to self, from navigator. As much as I want to do this, I'm sympathetic to w3ctag/design-principles#426 (comment), so for now I'm leaning towards not moving it. So I'll say that this PR closes #173.

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[domfarolino]

Hey @tabatkins, I could not for the life of me figure out how to add {{ModelContext}} or <code>ModelContext</code> to the actual linking text, so I had to settle on this super verbose [=foo|<code>foo</code>=] which is really redundant and I can't figure out if there's a way around it, so I can tighten these up. It's not blocking, so if there's no solution, that's fine.