Context will linger when navigating away from app

Now we correctly kill the context immediately when the user closes the last interest relevant to it.

Really though we only want to keep the lingering behavior for when the user navigates inside the same domain (and so the app will likely come back, so we don't want to tear down and set up the context again).

If we get a navigation event, to a different domain, we can go ahead and call context_destroy with the user_abandoned parameter set to TRUE.