FIX: show correct income from vehicles on screen

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6522 8aca7d54-2c30-db11-9de9-000461428c89
CODE: unify spieler_t::add_message and add_money_message interface

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6523 8aca7d54-2c30-db11-9de9-000461428c89