Upgrade app to scene-based life cycle

[herzbube]

iOS 13 started to support so-called "scenes". A scene is basically one instance of the app's user interface. An app may want to support different scenes, in order to show a different UI depending on the circumstance. Also an app may want to support showing multiple scenes at the same time (at the time of writing limited to iPad devices).

Up until iOS 26, scene support in an app was optional, but the next iOS version after iOS 26 will require apps to have scene support. [1] is a tech note providing a guide how to add scene support to a project. Additional useful documents are [2], explaining the different aspects of an app's life cycle that can be managed by an app delegate, and a scene delegate. Finally, [3] describes how to specify and set up the scenes in an app.

[1] https://developer.apple.com/documentation/technotes/tn3187-migrating-to-the-uikit-scene-based-life-cycle
[2] https://developer.apple.com/documentation/uikit/managing-your-app-s-life-cycle?language=objc
[3] https://developer.apple.com/documentation/uikit/specifying-the-scenes-your-app-supports?language=objc

[herzbube]

The "Screen Mirroring" feature on iOS devices creates a new scene with UIWindowSceneSessionRoleExternalDisplayNonInteractive.

See https://developer.apple.com/documentation/uikit/presenting-content-on-a-connected-display?language=objc