FIX: correctly top next window when closing the current top window

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6565 8aca7d54-2c30-db11-9de9-000461428c89
reduce code duplication

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6566 8aca7d54-2c30-db11-9de9-000461428c89
FIX: clear scenario string cache when starting new scenario

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6567 8aca7d54-2c30-db11-9de9-000461428c89
CHG: only update toolbars after finishing calls to script in scenario_t::step

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6568 8aca7d54-2c30-db11-9de9-000461428c89