Refactor out todo item serialization

Add a todo_write() function that allows one to serialize todo items and
write serialized data to an output stream in a fashion similar to
apoint_write() and event_write().

Signed-off-by: Lukas Fleischer <calcurse@cryptocrack.de>

src/calcurse.h

@@ -827,6 +827,7 @@ int           todo_hilt_pos (void);
 char         *todo_saved_mesg (void);
 void          todo_new_item (void);
 struct todo  *todo_add (char *, int, char *);
+void          todo_write (struct todo *, FILE *);
 void          todo_flag (void);
 void          todo_delete (struct conf *);
 void          todo_chg_priority (int);