Adds a drop-down list provider for the specified argument
void AddList( int argIndex, CXlWizExUIListProvider* plist );
The index of the argument. (The first argument has index 0.)
A pointer to a CXlWizExUIListProvider object.
Once the pointer in plist is passed to this method, the object pointed to is "owned" by the CXlWizExUIFunctionDescriptor object. The destructor for plist should not be called, since it will be called by the CXlWizExUIFunctionDescriptor destructor.