Add inline admin support!

Turns out to be really easy to implement,
as the TranslatableModelForm does all the form.
Moved the shared logic to an admin base class.

The current implementation does require the parent model to be a
TranslatableModel, as it's used to display the global language tabs.
The available languages of the parent model define which inlines are
displayed.
Also delete inline translations in delete_model_translation()
forms: avoid creating the empty object, allow inlines to show fallback title.
Support `translations = TranslatedFields()` on the base class.

This is currently needed in case the parent model has no translated
fields, but it still has inlines.
Merge branch 'inline_support'
Smallfix: self should be cls in get_translated_fields()