Dropped AppCache._empty, _with_app and _without_app.

It's now easier to achieve the same effect with modify_settings or
override_settings.
Imported override_settings from its new location.
Improved test isolation by resetting various caches.

Specifically attempted to reset properly all caches that store the
result of app_cache.get_app_configs() when INSTALLED_APPS changes.

Removed TransRealMixin which was an ad-hoc implementation of the general
mechanism provided by the setting_changed signal.