iOS 15 deprecation warnings: UIApplication.windows

[koke]

After merging #7851, which drops support for iOS 14, there are a few new warnings related to us trying to access UIApplication.windows.

This is mostly used on a reimplementation of currentKeyWindow, which was deprecated in iOS 13. We'll need to eventually invest some time in adapting the existing hacks to window scenes, or removing them altogether.

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[mokagio]

#8327 provides an attempt to silence the warning by updating the re-implementationt.

We'll need to eventually invest some time in adapting the existing hacks to window scenes, or removing them altogether.

Good point. @crazytonyli made a similar remark.

Maybe it's time to stop relying on the no longer true concept that there is one "key window" across the whole application?

I had a quick look at online and on the docs and found the following, which might be useful to start the work:

• So far, only iPadOS and macOS support multiple windows
• The concept of key window is still pretty much valid, as proved by the [keyWindow](https://developer.apple.com/documentation/uikit/uiwindow/1621612-keywindow?language=swift) property and [makeKeyAndVisible()](https://developer.apple.com/documentation/uikit/uiwindow/1621601-makekeyandvisible?language=swift) method on UIWindow. From the docs, "In iOS 15 and later, the value of this property is YES when the window is the key window of its scene. [...] Only one window at a time may be the key window." UIScene also has a keyWindow property.
• As this article points, accessing the first (or last) element cannot be trusted to give deterministic results