FIX crash with dragging tools over minimum speed sign

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6733 8aca7d54-2c30-db11-9de9-000461428c89
CODE: make marker singleton, do not store it in karte_t

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6734 8aca7d54-2c30-db11-9de9-000461428c89
CODE: add method to check and mark tiles at once

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6735 8aca7d54-2c30-db11-9de9-000461428c89
translate marker functions

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6736 8aca7d54-2c30-db11-9de9-000461428c89