[Dialog][AlertDialog] Enable outer scrolling dialog pattern

[benoitgrelard]

Fixes #887

I'll explain the thoughts I had and what I did here:

Problem

The main issue left with being able to create an outer scrolling dialog was that we have RemoveScroll in DialogContent, therefore making the overlay non-scrollable too.

Thoughts and solution

• The overlay is only used when it's a modal dialog, which incidentally is when we want to remove scroll (sure enough, we were doing this in DialogContentModal)
• So I moved RemoveScroll inside DialogOverlayImpl.
• The only remaining issue was that we also disable outside pointer events from DialogContentModal, so that was still preventing scrolling the overlay, for that I specifically re-enable them on the overlay.
• It also required moving the allowPinchZoom prop as it was previously on DialogContent. I remember almost raising this in the previous PR review that I thought perhaps it should live at the root as it's kinda an implementation detail that it was on the "content". I guess this solidified that feeling, we could have moved the prop to the overlay but it would definitely feel odd there I think.

---

Notes for docs:

• Make sure the allowPinchZoom prop is documented on the right part (Root).

[jjenzz]

I wonder if we should move this prop in the other components that support it too then?

[benoitgrelard]

Yeah, probably in a different PR.