Patch for plural forms

I write patch for plural forms of translated texts. It solves second
problem from report by Heikki:

http://archives.postgresql.org/pgadmin-hackers/2009-05/msg00017.php

It use "wxPLURAL" instead "_". I also change "stringextract" script to
know this. Following steps required when patche is aplied:
- regenerate POT template
- merge it with PO files
PO files must have set Plural-Forms correctly, e.g. for czech it si:
"Plural-Forms: nplurals=3; plural=(n==1) ? 0 : (n>=2 && n<=4) ? 1 : 2;
\n"
You can find it in gettext documentation (section 10.2.5) or create new
PO file from POT by msginit with appropriate param -l.

Marek