Don't call GObject.get_type() in ctor

Just experienced a crash in the SmtpOutboxEmailIdentifier
constructor trying to get its own type name.  Past experience
has shown that calling into GObject (or GType) in the constructor
before calling base() is dangerous, so changing this to a flat
string.

src/engine/imap-db/outbox/smtp-outbox-email-identifier.vala

@@ -8,7 +8,7 @@ private class Geary.SmtpOutboxEmailIdentifier : Geary.EmailIdentifier {
     public int64 ordering { get; private set; }
     
     public SmtpOutboxEmailIdentifier(int64 message_id, int64 ordering) {
-        base ("%s:%lld".printf(get_type().name(), message_id));
+        base ("SmtpOutboxEmailIdentifer:%s".printf(message_id.to_string()));
         
         this.ordering = ordering;
     }