added: mechanism to defer initial state read for sub apps

[HenrikJoreteg]

hey @jchip and @christianlent! I can't actually seem to get fyn bootstrap to run successfully, so i'm struggling to run tests, etc.

So i figured I'd at least send this PR to clarify what I'm suggesting we do.

Basically, this would allow us to run instance.getInitialState() and thereby defer the reading of that data instead of blocking on that with the inline script (as we discussed).

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[HenrikJoreteg]

there's a really good chance this breaks other things 😅 . I'm mainly proposing an idea here, not necessarily saying this is merge-worthy.