Restore macOS <Image> and ActionSheetIOS. Cherry pick multi windows UIScene support from upstream.#256

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tom-un merged 3 commits intomicrosoft:masterfrom

tom-un:tomun/imageview

Mar 7, 2020

Commits on Mar 6, 2020

Commits on Mar 7, 2020

release _window now to ensure its UIKit ivars are dealloc'd on the main thread as the RCTRedBox can be dealloc'd on a background thread
tom-un
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