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/todo.c

@@ -183,6 +183,15 @@ todo_add (char *mesg, int id, char *note)
   return todo;
 }
 
+void
+todo_write (struct todo *todo, FILE *f)
+{
+  if (todo->note)
+    (void)fprintf (f, "[%d]>%s %s\n", todo->id, todo->note, todo->mesg);
+  else
+    (void)fprintf (f, "[%d] %s\n", todo->id, todo->mesg);
+}
+
 /* Delete a note previously attached to a todo item. */
 static void
 todo_delete_note_bynum (unsigned num)