Add missing stuff

Added assorted bits that I'd found myself missing:

AbstractField.valid

AbstractSelect.setItemCaption: Don't know if this should be AbstractSelect.itemCaption, but it felt like I should prepend "set" because it isn't a single variable. Maybe it'd be better modeled as a Map[Any, String] but I don't know enough about Scaladin internals to pull that off.

Select: I was surprised that there was a NativeSelect component but not a plain Select.