Use Auto-Submitted: auto-generated header to identify bots

New `dc_msg_is_bot()` C API and corresponding `Message.is_bot()`
Python API can be used to check if incoming message is sent by a bot,
e.g. to avoid two echo bots replying indefinitely to each other.

"Bot" flag is not set for outgoing messages, but may be set for
BCC-self messages. For now documentation says that `dc_msg_is_bot()`
return value is unspecified for outgoing messages. It can be better
specified later if needed for specific applications, e.g. sharing an
account with a helper bot.

src/mimefactory.rs

@@ -491,6 +491,11 @@ impl<'a> MimeFactory<'a> {
                 "Auto-Submitted".to_string(),
                 "auto-replied".to_string(),
             ));
+        } else if context.get_config_bool(Config::Bot).await? {
+            headers.unprotected.push(Header::new(
+                "Auto-Submitted".to_string(),
+                "auto-generated".to_string(),
+            ));
         }
 
         if self.req_mdn {