Allow 'translation_domain' attribute on validators

This change allows the ```translation_domain``` attribute on ```validators``` elements. The child validator elements inherit the translation domain of their parent. This change prevents developers from repeating the same ```translation_domain``` attributes on many consecutive validators that often share the same translation domain anyways in their context.

Example:
```xml
<validators method="write" translation_domain="default.error">
    <validator name="email" class="email">
        ...
    </validator>
</validators>
```

The validators (like 'email' above) can of course still have their own translation domain deviating from the default of their parent element. Comments welcome (e.g. line 218).