Replace Geary.Account::send_email w/ Smtp.ClientService methods

Remove Geary.Account::send_email and the sending progress monitor in
favour of exposing Geary.Smtp.ClientService as a public class and so
allowing clients to acccess those symbols from Geary.Account::outgoing.

Also make ::send_email a convenience class for doing a save/queue and
expose those operations, so clients can managage the two seperately if
desired (e.g. for undoing sending).

Finally, make Outbox.Folder public since it's exposed by the SMTP
service and maybe clients want to do something more detailed with it
anyway?

test/engine/api/geary-account-mock.vala

@@ -181,12 +181,6 @@ public class Geary.MockAccount : Account, MockObject {
         );
     }
 
-    public override async void send_email_async(ComposedEmail composed,
-                                                Cancellable? cancellable = null)
-        throws Error {
-        void_call("send_email_async", {composed, cancellable});
-    }
-
     public override async Gee.MultiMap<Email,FolderPath?>?
         local_search_message_id_async(RFC822.MessageID message_id,
                                       Email.Field requested_fields,