added level editor option for buggy BD style conveyor belts

added level editor option for buggy BD style conveyor belts

This makes the change from the last commit optional in the level
editor, deactivating it for existing levels in GDash formats (and
therefore also replays), so neither existing levels (caves) nor tapes
(replays) will be broken anymore by that change. For newly created
levels in R'n'D format, however, this change will be activated by
default, using the new, fixed conveyor belt behavior.