Ajout du support Gettext/en

Gettext is a program intended for the management of languages in computer applications. Program coming from the GNU project, It is very used in freeware, command line applications, graphical user interfaces and web sites. Gettext is a set of programs and a "C" API.

For more information:
 * Gettext manual.
 * Wikipedia fr and en.

For the programmer
The functions of the gettext "C" API are available by including the file i18n.hpp in the directory vle/utils/i18n.hpp

In the VLE source code source, the sentences to translate have to be written in English and be put in a call to the gettext function :

For the translator
Creation of the catalog from all the C++ files and Glade :

Creation of a french translation file of translation:

Compilation of a catalog (build by the Gettext script of CMake):

Update of a translation file with one new release of the catalog file: