Adding more classes (including support for config-based facet groups)

This is related to https://github.com/GigaOM/legacy-pro/issues/4155, but doesn't need to wait for the actual styling to get pushed.

Done with @Camwyn
This could be avoided if the title class were always the facet name.

But then you have to explain why that has more value than the `'facet-' . $instance['facet']` widget class you have above.