facet should come from the layer, not __launchpad_facetname__

[Curtis Hovey]

The facet menu works by reading the __launchpad_facetname__ attribute of the view. The publisher adds that attribute to the view if the view does not have it. If the facet menu is passed a context object, there is no view, so there is no facet set.

The attribute is associated with a vhost's request type. That request object has a layer. The facet menu should use the layer associated with the request. We require a mapping or an adapter to convert the layer to a named facet. I image that the LaunchpadRequest's layer is synonymous with the Overview facet in the menu.