FIX: call is_scenario_completed only from scenario_t::step

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6268 8aca7d54-2c30-db11-9de9-000461428c89
move export of open_info_win to api_gui.cc

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6269 8aca7d54-2c30-db11-9de9-000461428c89