Incompatibility with django-nested-inlines fixed

# Problem
When using https://github.com/Soaa-/django-nested-inlines, the tabular tabs on the second nested level are broken. The tabs are only showed for the last item and the table layout breaks.

![bildschirmfoto 2013-11-27 um 13 51 35](https://f.cloud.github.com/assets/569215/1631358/dd81e3a0-5762-11e3-8c23-e027925e9e77.png)

# Reason
This is due to TranslationField#buildGroupId dropping those fields as it doesn't understand their ids.

# Fix
Handle nested inline ids explicitely.