Use e_source_camel_generate_subtype() to register an ESourceCamelNone
class based on the newly-registered "none" CamelProvider.

e_source_camel_register_types() won't pick up the new provider if it was
already called prior to the provider being registered.  That might be a
bug... I'm not sure yet.  The Camel/ESource integration is kinda messy.