[AppKitBackend] Implement Spacer

[stackotter]

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Currently there's a dummy spacer implementation so that examples which rely on Spacer can at least run, but it doesn't actually do anything. I had a look around and couldn't find any good resources on making a Spacer-like view with AppKit (the concept of 'expanding view' doesn't seem like a thing?). Maybe an AppKit guru out there can help come up with a good solution!