All entity id to be displayed vi RooToString

I believe @RooToString currently ignores the id field generated by @RooEntity.
However, this field is extremely useful for debugging or for the web mvc scaffolding UI.

Please allow @RooToString to generate this id.

For backward compatibility, you could update
String[] excludeFields() default "";
to exclude the id by default.

However, I would recommend including it as default.