The Library package offers the PassepartoutImplementations target for
OpenVPN/OpenSSL and WireGuard/Go, but it doesn't need it itself. Only
the main app does, so move the dependency there.
On the other side, drop the potentially problematic AppUI meta target.
Move platform filters to the Xcode project.
Indirectly fixes a crash with Xcode 16 Previews on iOS (forced to use
legacy previews before):
https://forums.developer.apple.com/forums/thread/756681
- Refactor AppUI initialization in all platforms (sort of template
method pattern)
- Make AppMenu specific to macOS by wrapping it into a folder for
consistency
- Add SizeClassProviding for repeated checks on hsClass/vsClass
Fixes#659
Davide