Add TranslateableProperties

The `TranslateableProperties` mixin acts like the `TranslateableAttributes` mixin but sets the translated values on the object itself instead of as DOM attributes.

I considered making the translations computed properties that use the object itself as the context for the translation, but I couldn't think of a way (other than making them all `volatile`) of expiring the computed translations. I'm open to the idea, but not yet sold.