The unification of the savegame and loadgame dialog in rP22923 / D2290
introduced the new SavegameWriter class, but didn't change everything to classes in that folder,
while having moved the former gui/common/functions_utility_loadsave.js without deuglifying it,
while leaving one unused function (getSavedGameData()) and one wrong reference saveGame()).
This patch rewrites everything to use the OOP style with the classes keyword.