csstypes: Change NTH_CHILD and NTH_LAST_CHILD change propagation
For now assume that if those flags change for a node, they also changed for all sibling nodes. This is not strictly true but simplifies invalidation. The reason it simplifies invalidation is that when removing or adding a node, we can just invalidate the first node (for NTH_LAST_CHILD) and the new node (for NTH_CHILD) and all the other nodes will be invalidated automatically.
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