CamelStoreClass bumps the settings_type to CAMEL_TYPE_STORE_SETTINGS,
which has a filter-inbox property. But we don't want ANY settings in
a CamelNullStore, so revert the settings_type to CAMEL_TYPE_SETTINGS.
Evolution kinda sorta supports multiple identities by allowing users
to set up so-called "transport-only" accounts by choosing "None" for
the account type.
Add a CamelStore subclass for those types of accounts so they get
added to EMailAccountStore. It's just a stupid hack to keep another
stupid hack working, but this should sustain us until we can support
multiple identities FOR REAL.
