Plural strings

Hi !

Here is the patch to handle plural forms.
By now,it works only if the PO file already has a valid Plural-Forms handler.
Refer to your GNU gettext doc for details

When a string with plurals is found, you get :
the plural form in the "original string" edit (separated by a '---' line )
and as many edits as you need based on your "nplural" value.
That's 2 in english and french.

What's missing is filling out correctly the "Plural-Forms" field when I 
have only gettext "placeholder".
It's the next move from here.
I wonder if I should put the known forms in an external file !?

I think I've modified any piece of code that needed it (except for TM where 
things are "fuzzy" for me) but may be I'm wrong...
If you can do some tests I'd love some feedback good or not !

About HTML export :
When I thought about plural strings I realized that it would impact the 
export code so I prefer to finish plural strings first !


Regards,
Christophe.