Seems like the natural fit, since we are already adding other XML bits
in the PostParse routine.

Previously AddImplicitControllers was only called at the end of XML
parsing, meaning code that builds a DomainDef by hand had to manually
call it. Now those PostParse callers get it for free.

There's some test churn here; xen xm and sexpr test suite bits weren't
calling this before, but now they are, so you'll see new IDE controllers.
I don't think this will cause problems in practice, since the code already
needs to handle these implicit controllers like in the case when a user
defines their own XML.