Strings used with add_quest()/update_quest()/end_quest() in dialogs and events
are translated at load time. They are thus saved translated.

This mean that if a player changes the current language, the quest log messages
already stored will not be displayed in the new selected language.

Translation of the quest log messages has to be done when the quest log is
displayed, not at load time. 

But, some texts are in dialogs (po-dialogs), others are in events (po-data).
As a consequence, they are in 2 message domains and we can not use one of our
gettext() macros. 

Marked texts in events.dat are then to be transferred into the
freedroidrpg-dialogs domain. We postpone the fix of this bug until events.dat is
lualized.