* add support for message-references:

- mu_msg_get_references and mu_msg_get_references_str get the list of
    refences ("References:" and "In-reply-to:") as a GList,
    resp. comma-separated list
  - stored in database as a value, can be shown in mu find output using 'r'
    (db needs rebuild)
  - document in mu-find manpage

src/mu-msg-fields.h

@@ -41,7 +41,8 @@ enum _MuMsgFieldId {
 	MU_MSG_FIELD_ID_TO,
 	MU_MSG_FIELD_ID_MSGID,
 	MU_MSG_FIELD_ID_TIMESTAMP,
-
+	MU_MSG_FIELD_ID_REFS,
+	
 	MU_MSG_FIELD_ID_NUM,
 
 	/* fake fields */