Display multiple elements in section headers

[dfabulich]

We're adding a new environment variable so the grid/stack/list can convey an axis to the Section, so it knows whether to make a VStack or an HStack.

(Should I be using a SkipUI VStack/HStack for this…? Is there a better way?)

Fixes #311

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[aabewhite]

I took a look at this, and I think there's a better/simpler way to do it. But in the process of investigating I found a bug in our header/footer logic. Right now it's minor and only affects spacing of the top header. But it also would prevent my idea from working. So I'd prefer to wait on this until I have time to both fix the bug and then apply my idea