Use builtin functions for city growth

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@7137 8aca7d54-2c30-db11-9de9-000461428c89
Correctly calculate offset for tab and more similar name for tab header

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@7138 8aca7d54-2c30-db11-9de9-000461428c89