public TextSpinner<T> setTextDisplayMapperPure(@Nullable final Func1<T, String> textDisplayMapper) {
        return setTextDisplayMapper(textDisplayMapper == null ? null : v -> TextParam.text(textDisplayMapper.call(v)));
    }
if spinner is be represented as a textview, set how currently selected value is displayed
If not set, then mapper set with {@link #setDisplayMapperPure(Func1)} is used
main/src/main/java/cgeo/geocaching/ui/TextSpinner.java