Add a default value placeholder for inherited classprefs

[rob]

Currently, to specify a default value for a classpref that is defined in a superclass, you have to redefine the classpref itself. This is not so bad for int, bool, etc, but for lists that means you have to redefine the list definition. That is bad.

Should instead have a default value param that just replaces the default value in the subclass without changing the definition of the superclass.