ios: support the UIScene lifecycle

[molenick]

Adds support for the UIScene lifecycle on iOS, unlocking winit support for apps built with the iOS 27 SDK.

iOS 27 refuses to launch apps built with its SDK unless they adopt the UIScene lifecycle by declaring a scene manifest in Info.plist. In apps that declare one, winit shows a blank screen: it creates its window before a scene exists and never attaches it to one.

Related: #4224

Changes

• In apps with a scene manifest, can_create_surfaces is emitted once the first scene connects, instead of at launch.
• New windows are attached to the connected scene.

Impact

• Apps can declare a scene manifest and run on iOS 27. No scene delegate is needed.
• Apps without a manifest are unaffected.
• To preserve compatibility with older iOS versions, UIScene symbols are looked up at runtime instead of linked.

Testing

I wrote XCUITests that launch a winit app and check that its window appears, run with and without a scene manifest on the iOS 17, 26 and 27 simulators.

• With a manifest, the window did not appear on iOS 17, 26 or 27 before this change, and now appears on all three.
• Without a manifest, nothing changes: the app works on iOS 17 and 26, and iOS 27 refuses it as expected.

tvOS and visionOS are untested.

A backport to 0.30.13 is on uiscene-minimal-v0.30; happy to open a PR against v0.30.x if wanted.

• Tested on all platforms changed (iOS simulator)
• Added an entry to the changelog module if knowledge of this change could be valuable to users
• Updated documentation to reflect any user-facing changes, including notes of platform-specific behavior
• Created or updated an example program if it would help users understand this functionality