I propouse some changes in header.ch

[Nacho <nacho resa gmail com>]

Hi folks,

Working in plurals stuff I realized about several things that
with the current implementation of Pablo is quite easy to
change.
* First thing: We have to manage all related things with 
header internally in our header class.
* We have to modify the header in the moment we want
to add something to the header class. Why?
Because for example: Gtranslator right now detects all
problems/errors in a po file. If have a message with plurals
and we don't have a plural form in our header we can't following
editing our po file. So we have to store the plural string in the
moment the user add a one to the preferences.
* Finally: I think this can do easier the implementation.

It is just a propouse, so what do you think?