* 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-priv.h

@@ -43,6 +43,8 @@ enum _StringFields {
 	MDIR_FIELD,        /* the maildir */
 	
 	FLAGS_FIELD_STR,   /* message flags */
+
+	REFS_FIELD,        /* msg references, as a comma-sep'd string */
 	
 	FIELD_NUM
 };
@@ -59,6 +61,8 @@ struct _MuMsg {
 	size_t		_size;
 	time_t		_timestamp;	
 	MuMsgPrio       _prio;
+
+	GSList          *_refs; /* msgids of message we refer to */
 };
 
 G_END_DECLS