macos: break reference cycle to window to allow window to free memory

[mitchellh]

Fixes #366

The comment in the Swift code explains what was happening here:

I don't know if this is the right place, but because of WindowAccessor in our
SwiftUI hierarchy, we have a reference cycle between view and window and windows
are never freed. When the window is closed, the window controller is deinitialized,
so we can use this opportunity detach the view from the window and break the cycle.

An alternate solution would be to make our reference back to the window "weak" but we appear to not be able to do that with SwiftUI property wrappers such as @State and @Binding and so on.

Before

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After

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