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.

python/src/deltachat/message.py

@@ -141,6 +141,10 @@ class Message(object):
         """ return True if this message was encrypted. """
         return bool(lib.dc_msg_get_showpadlock(self._dc_msg))
 
+    def is_bot(self):
+        """ return True if this message is submitted automatically. """
+        return bool(lib.dc_msg_is_bot(self._dc_msg))
+
     def is_forwarded(self):
         """ return True if this message was forwarded. """
         return bool(lib.dc_msg_is_forwarded(self._dc_msg))