It's been years; I guess the chance of Apple's AppKit/ObjC on Windows becoming public is basically zero at this point.
On the mac, the basic system libraries are broadly split in to AppKit and Foundation (+ other stuff). Foundation is cross-platform, the source is already available, and @compnerd has done a lot of work porting it to Windows. AppKit, on the other hand, is generally not portable; it contains a lot of things specific to macOS, which might not even have equivalents on other platforms, or if they do they might look/behave differently. The thing that iTunes uses is probably only a subset of AppKit. Trying to squish one platform's concepts on to another is one of the reasons bringing UIKit to the mac has proven so controversial. Besides, it's not even clear what the future of AppKit is on the mac; it might all just be UIKit in the future. What I'm saying is: don't hold your breath. (just to make clear, because there are a lot of employees on these forums: I don't work at Apple).
Even though it's not an ideal solution, Microsoft did try and bring these libraries to Windows anyway as part of Objective-C for Windows, using the GNUStep runtime. We've talked about bridging with that runtime before, and I think the consensus was that the project maintainers aren't opposed to it, but also aren't interested in implementing/maintaining it themselves.