Underlay?

[makoConstruct]

It's fairly common for apps to want to place a visual indicator underneath the selected widget (and its siblings) rather than over it. Two examples are shown here flutter/flutter#166792

Describe the solution you'd like
I think the right api would be

Portal(
  underlay: true,
  child: ...,
)

Where underlay places the portalFollowers behind child instead of over it.

Describe alternatives you've considered
It's not quite addressed by #45 , since this only seems to support controlling which ancestor the portal projects into, and any ancestor will render over the follower widget. But it does depend on 45, since it's going to be common for a user to want both an underlay portal and an overlay portal and need to be able to choose between them.

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If there are obstacles to implementing this, I'd be curious to hear them, as it's relevant to flutter/flutter#166792

[fzyzcjy]

Looks interesting, feel free to PR for this!