admin: added inline support if the parent model is not a TranslatableModel

When the parent model isn't a translated model, the tabs will be
displayed on the inlines themselves (which also requires a different
template). NOTE: there is no support for the delete_translation view
yet, hence the delete buttons are disabled for inline tabs..
API: add language_code parameter to safe_translation_getter()

Allow fetching a single field from a different language.
This can be useful when switching the object language isn't possible.
For example, because it would affect an object that is shared between threads.
updated CHANGES