[AppKitBackend] Fix padding modifier

[stackotter]

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ControlsExample currently has uneven spacing between the three groups when run using the AppKitBackend (SCUI_BACKEND=AppKitBackend swift run ControlsExample). It seems like .padding(.bottom, 20) is adding 20 pts of padding to both the bottom and the top of the element (removing the padding on the bottom of the first group fixes the spacing). This effect doesn't seem to happen for leading and trailing padding (only top and bottom).