Allow globally overriding slots in the light dom of components and use it to override the auth-info slot in the Nextcloud file picker

This is a bug, but the mixin just ignored undefined here.
The next version will be stricter and raises an error in this case.

Create a new instance with createInstance() and call setOverrides()
with override data to override translations.

See the tests for some examples.

Uses a separate namespace that gets used over the default one, so we can override,
but also remove the overrides again when needed.

This has the problem that all clones have the same store, so this overrides
the translations for all clones and not just the one providing the overrides.