reorder gettext.hpp

I think that libintl/gnugettext has no support for unicode filenames on windows. Thats why i want to replace it with boost locale, at least for our windows builds.

The the plan is to move all dependencies of libint.h to gettext.cpp and then add an alternative implementation gettext_boost.cpp. The boost gettext version return std::strign in the default  implementation, that why i changes the gettext.hpp functions to return std::string.

i want to put add "libintl.hpp" into the gettext.cpp file
I assumes that it doesnt matter whether i have # define GETTEXT_DOMAIN "wesnoth" or not since that's the default textdomain so i added a 
```
#ifndef GETTEXT_DOMAIN
# define GETTEXT_DOMAIN PACKAGE
#endif
```