RFC-24 for symbolObjs is done

[Umberto Nicoletti]

As I promised last friday I have worked on symbolObjs to implement
rfc-24 and now the code is in svn (rev. 6308).

The code compiles cleanly (including mapscript and php), is memory
leaks free (tested with valgrind) and runs the msautotest suite
without errors (segfaults, that is).
I have, as usual for RFC-24, added new tests to the refcount.py python
test tuite.

I still have to update the rfc page, I'll do that later.

Regards,
Umberto