fix: Make sure children are requested for new expanded parents

[tomivirkki]

Fixes #2055

[yuriy-fix]

There is one undesirable side effect:
In Grid flow there is a method that allows to check this fix: scrollToEnd.

With this change it won't actually scroll to the end as _effectiveSize is not yet up to date.

[tomivirkki]

There is one undesirable side effect

I don't think it's related to this change (happens even with the linked project without any changes).

But that said, this PR doesn't actually resolve the original issue even though it claims to fix it. I think I'll add another change to the PR so it also continues scrolling after new items get loaded.

[tomivirkki]

Done.

[tomivirkki]

Reverted the latest change based on follow-up discussions. Having the grid automatically continue scrolling after child items (and possibly their child items) are received from the server, might be unexpected for the user. Especially with slow connections.

We also discussed having a separate feature, say grid.stickToBottom = true, that would enable the grid to always stick to the bottom scroll position on size changes.