CODE: unify message box and info windows, re-factor common code into base class

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@7189 8aca7d54-2c30-db11-9de9-000461428c89
Enable scaling via theme.dat for more buttons

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@7190 8aca7d54-2c30-db11-9de9-000461428c89