TTD stores invalid orders different than OTTD, this resulted in empty lines in the order list. With the overhaul of the order system this got worse: no line was shown at all.
Fix this by sanity checking while loading and convert the orders accordingly.
This adds no functionality, but is a stepping stone for future improvement (like 16bit order indices) and is easier to read.
This changes preserves binary compatibility wrt savegames.
-Fix: Sorter icon pointing down 'v' sorts in every window lowest value first, '^' highest value first
-CodeChange: move Dropdownlist from settings_gui.c to widget.c. More in place there.