wkt-excel: Download in multiple formats (eg raw entities, and discovered DTOs) with a selector

[danhaywood]

from https://github.com/isisaddons-legacy/isis-wicket-excel/issues/4

introduce this into Isis applib:

public interface Mapper<S,T> {

```
Class<? extends S> getSourceClass();
Class<? extends T> getTargetClass();

@Programmatic
T map(S s);
```

}

Then search for domain classes that implement:

@DomainService(nature=DOMAIN)
public CountryToDto implements Mapper<Country, CountryDto> {
   public CountryDto map(Country c) { ... }
}

@DomainService(nature=DOMAIN)
public CountryToRowHandler implements Mapper<Country, CountryRowHandler> {
   public CountryRowHandler map(Country c) { ... }
}

and provide a drop-down selector to choose how to export (as Country, as CountryDto, as CountryRowHandler etc).

```
```