UP-4305 : Add parameter target so we can have specific portlets launc...

...h in a new window (or target window)


@drewwills note that this `@Override` is on the `MissingPortletDefinition` (implements `IPortletDefinition`) private static final inner class, rather than on `RDBMDistributedLayoutStore` itself.  Goodness knows `RDBMDistributedLayoutStore` could use refactoring -- but I don't think the strewn about trivial implementations of `IPortletDefinition` should impair adding useful syntactic sugar to the interface. :smile: