reorder gettext.hpp

We want to put all dependencies of linintl.h to getttext.cpp so we can easily replace gettext/libintl with boost locale gettext by replacing that file.
make gettext.hpp return std::string.

in 99% of cases the returned value was moved into a std::string anyway. We do this becasue the boost gettext functions also return std::string so it makes replacing them easier.
use boost locale gettext

This includuces an alternative implementation of gettext.cpp. Libintl gettext doesn't seem to support utf8 filenames on windows. Also it causes problems when changign the language on my msvc build. That why i want to use boost locale gettext at least for our windows builds.
This requires boost locale which is introduced in boost 1.48 (That's why i still hesitate to make it the default on all systems).
fix unused warnings.
fix compilation

it seems like sometimes libintl.h is needed for LC_MESSAGES so we need
to move that last code that uses LV_MESSAGES out of language.cpp into
gettext.cpp
fix comilation

move LC_MESSAGES out of wesnoth.cpp and game_config_manager.cpp
fix gettext_boost.cpp
fix image translations