Bug 4154 - Add a .po based mechanism to translate preferences

Bug 4154 - Add a .po based mechanism to translate preferences

Add to previous patch (and replace it):
  - update function
  - translation of tab subsection labels
  - GPL2
  - fix last minute bug catched by Galen

Cut-and-past of pref-trans script perldoc:

NAME
       pref-trans - Handle preferences translation

SYNOPSYS
         pref-trans init fr-FR
         pref-trans update fr-FR
         pref-trans install fr-FR

USAGE
       pref-trans init lang
           Create a .po file in po directory, named lang-pref.po. This
           file contains text to translate extracted from .pref files.

       pref-trans update lang
           Update a .po file in po directory, named lang-pref.po. This
           file contains new text to translate extracted from .pref files.
           Previous translated text are kept. There is a minor bug, which can’t
           be fixed due to preferences data struture: preferences tab
           subsection labels are lost when updating .po file.

       pref-trans install lang
           Use lang-pref.po file to translate the english version of
           preferences files and copy those files in the appropriate
           directory.

DESCRIPTION
       Koha preferences are stored in a data structure found in
       koha-tmpl/intranet-tmpl/en/module/admin/preferences/ files.
       Depending of user language, other files are used. This script extract text
       from ’en’ preference files, and put them in one .po file.  This .po
       file can be updated. When completed, a .po file can be applied to create
       localized versions of preferences templates.

Signed-off-by: Galen Charlton <gmcharlt@gmail.com>