* mu-msg-part: add mu_msg_part_save_temp and some refactoring

src/mu-msg-part.h

@@ -120,6 +120,21 @@ gboolean mu_msg_part_save (MuMsg *msg, const char *filepath, guint partidx,
 			   gboolean overwrite, gboolean use_cached, GError **err);
 
 
+/**
+ * save a message part to a temporary file and return the full path to
+ * this file
+ *
+ * @param msg a MuMsg message
+ * @param partidx index of the part to save
+ * @param err receives error information if any
+ *
+ * @return the full path to the temp file, or NULL in case of error
+ */
+gchar* mu_msg_part_save_temp (MuMsg *msg, guint partidx, GError **err);
+
+
+
+
 /**
  * get a filename for the saving the message part; try the filename
  * specified for the message part if any, otherwise determine a unique