Enable translation context for values of select attributes

Let's enable translation of select attributes, by introducing the `SelectAttributeValue` translation context.

Allows switching from this
![sav-0](https://f.cloud.github.com/assets/928116/1071793/02ae89ac-147f-11e3-89e6-b2fcbdd6844c.png)

To this
![sav-1](https://f.cloud.github.com/assets/928116/1071794/0d45c2b8-147f-11e3-8735-600472cda47b.png)

(please note that the attribute values are translated)