* 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.c

@@ -158,6 +158,13 @@ static const MuMsgField FIELD_DATA[] = {
 		MU_MSG_FIELD_TYPE_TIME_T,
 		"timestamp", 'x', 0,
 		FLAG_GMIME 
+	},
+
+	{ 
+		MU_MSG_FIELD_ID_REFS,
+		MU_MSG_FIELD_TYPE_STRING,
+		"refs", 'r', 'R',
+		FLAG_GMIME | FLAG_XAPIAN_VALUE
 	}
 };