Sets or returns the actual value of an argument (Variant). When you define
an argument (before XSIApplication.AddCommand is called), this property
represents the default value of an argument. This is persisted as part of the command
definition. This property can also be used to change the default value when you call
Command.Update.
When you use Command.Execute to invoke a command, you can provide an
actual value for this argument, which overrides the default value, but does not affect the
persisted command definition.
Warning: The Value is erased if Argument.Handler is called to set an
Argument Handler after the Value has been already set.