Support readonly_fields in tabbed translation admin

[deschler]

Add support for readonly_fields in tabbed translation fields.

This turns out to be not as easy as it sounds. ModelAdmin doesn't render readonly fields as form fields. As tabbed_translation_fields.js is based on gathering the form field name attributes and the custom css classes added by TranslationAdmin.patch_translation_field, the readonly fields aren't taken into account.

First attempts where made in the tabbed-readonly-fields branch, but these are unlikely to get merged into master. Too much hackery involved.

[ppython]

Anyone wants to attempt a new approach to this problem !?