Wizard attributes (readonly, required, invisible) now controlled
by 3 virtual fields each, allowing far greater control.  This fixes
a number of issues:

* Attributes with conditional domains might be required, or might
    not be
* Attributes with conditional domains marked as required can now
    be validated if the domain is empty.
* Readonly values can be better detected when the values are not
    passed from the front end.
* Super complex domains can now be used to drive the readonly
    and required values, ands and ors and inherited, as the
    driver is now purely the list of valid ids.
* Easier modification of behaviour in custom circumstances
    with inheritance possible to drive these virtual columns.