* NOTIFICATION_MOVED_IN_PARENT makes node children management very inefficient.
* Replaced by a NOTIFICATION_CHILD_ORDER_CHANGED (and children_changed signal).
* Most of the previous tasks carried out by NOTIFICATION_MOVED_IN_PARENT are now done not more than a single time per frame.
This PR breaks compatibility (although this notification was very rarely used, even within the engine), but provides an alternate way to do the same.
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