Subject: Node::remove_child() is called carelessly

A couple of recent bugs have been caused by remove_child() removing a
node from the parent_set when it shouldn't have, due to a sibling
using the same node.  One case was with two layer parameters linked to
each other, and the other was two sub-parameters of the Switch
valuenode being linked to each other.

This same type of bug seems to be present in other places too, and
should be killed.

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The two fixed bugs were:

2353284: If two parameters in the same layer are connected to the same
r2291	 valuenode and we disconnect one of them, the parent-child
	 relationship for the remaining connection was being deleted.
	 Now we search the parameter list to see if another parameter
	 uses the same valuenode.

2412072: Don't remove a node from the parent set of a linkable
r2311	 valuenode when replacing a link if the link is still in use
	 by one of the sibling links.