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.