unnecessary use of FacesContext in SkinProvider API

SkinProvider API uses FacesContext in its methods. All that is done inside the API is to extract the ExternalContext. So it is sufficient to pass ExternalContext to the API. Though this is a public API change, the SkinProvider API is introduced recently and not widely used. This gives us a chance to correct the API now.

Proposed change:

• public Collection<SkinMetadata> getSkinMetadata(FacesContext context)
  + public Collection<SkinMetadata> getSkinMetadata(ExternalContext context) { return Collections.emptyList(); }

• public abstract Skin getSkin(FacesContext context, SkinMetadata skinMetadata);
  + public abstract Skin getSkin(ExternalContext context, SkinMetadata skinMetadata);

and the related internal API changes.