Builtin functions on Changeable should be referenced explicitly

What new or enhanced feature are you proposing?
The "can cast?" properties on Changeable are not defined explicitly (e.g. 
Changeable.show), but instead defined through VisibleThing.functions. This 
reference should be made explicit.

What goal would this enhancement help you achieve?
Unification of operation modelling styles. This may either increase or decrease 
the complexity of modelling. It would allow for a list of builtin operations to 
be generated automatically through Modeldoc.

This is similar to issue 265.

Original issue reported on code.google.com by soundasleep on 5 Jul 2011 at 6:43

soundasleep / iaml

Builtin functions on Changeable should be referenced explicitly #267