We should consider slot as a parent in ChildIterator: if slot's
`assignedNodes` is not empty, use `assignedNodes`, otherwise, use direct
children as fallback content.
Also, GetFlattenedTreeParentNodeInternal should be changed to use
`assignedSlot` instead of `DestInsertionPoints`.
Set and unset assignedNodes properly when slot is added/removed and when
slotables are added/removed/modfied. Note that assigedNodes with
flatten: true is calculated on demand.
