Remove deprecated Info.plist key UIRequiresFullScreen

[herzbube]

The Info.plist key UIRequiresFullScreen is deprecated. Study [1] to figure out what needs to be done to be able to remove it. Adopting the scene-based life cycle (#434) may be a prerequisite.

[1] https://developer.apple.com/documentation/technotes/tn3192-migrating-your-app-from-the-deprecated-uirequiresfullscreen-key

[herzbube]

With iOS 9 the iPad received the first capabilities for multitasking: "Slide Over", "Split View" and "Picture in Picture". In #304 some work for supporting these capabilities was started, but never completed. Instead the Info.plist key UIRequiresFullScreen was introduced, to "temporarily" disable iPad multitasking capabilities. That was in September 2016, more than 9 years ago.

Since UIRequiresFullScreen is now deprecated and Apple threatens to remove support for it, the work in #304 must be continued. Once that issue is closed, the issue here can be closed as well.