FIX: do not cut productivity when placing fields for industries with random initial production

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6270 8aca7d54-2c30-db11-9de9-000461428c89
FIX: buttons in tab panel should not disappear if offset_tab > 0

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6271 8aca7d54-2c30-db11-9de9-000461428c89
CHG: Loading screen reworked.

git-svn-id: svn://tron.homeunix.org/simutrans/simutrans/trunk@6272 8aca7d54-2c30-db11-9de9-000461428c89