Instead of branching on the Node type, let subclasses decide how their
layout nodes get constructed.
This will allow elements to create custom layout nodes if they want.
This was a workaround to be able to build on case-insensitive file
systems where it might get confused about <string.h> vs <String.h>.
Let's just not support building that way, so String.h can have an
objectively nicer name. :^)
