Make usage of choice_translation_domain option at choice_widget_options block in a BC way.

I believe this fixes the translation handling for 2.7 referred at #377.

@florianeckerstorfer Did you have a chance to look into this?

It's basically the same as #378 but in way that should support SF < 2.7 as well.

+1
Using this bundle, a lot of "missing messages" appear in the dev toolbar even when using false as choice_translation_domain. So i'm unable to see if i really have missing translations or false positives only.