Refactor translation options to be able to keep track of translation fields inherited from concrete supermodels and find fields inherited indirectly (through unregistered parents).
localized_fieldnames_rev is no longer available, but should be replaceable by field.translated_field.
Manage translation fields inherited from concrete supermodels in admin.
Allow TranslationAdmin to be used with unregistered models.
Small cleanup in FieldsAggregationMetaClass.
Ensure that a base is not registered after or unregistered before submodel.
Adapt management commands and a debug message.
Separate registry for manager shouldn't be needed any longer -- the main registry caches options for all seen models.
Merge branch 'feature/complex-inheritance' into develop

Conflicts:
	modeltranslation/translator.py
Merge branch 'feature/temporarily-disable-fallbacks' into feature/filtering-with-fallbacks
Written a method rewritting lookups as fallback-languages-aware and added a `fallbacks()` toggle on manager to enable it on demand (such rewriting may be too heavy to apply to all queries, especially with long fallback chains).
Merge branch 'feature/filtering-with-fallbacks' into develop