[AppKitBackend] Stop sliders jittering in RandomNumberGeneratorExample

[stackotter]

The minimum slider's value is forced to be less than the maximum slider's value, and the maximum slider's value is forced to be greater than the minimum slider's value. In SwiftUI this approach works and the slider's range gets limited, however in SwiftCrossUI under the AppKitBackend the sliders jitter back and forth quickly when you slide them past the artificial min/max limits.