one MoveToNextItem() in DeleteVehicleNews (added in r3757). To work correctly do not
reset _forced_news to INVALID_NEWS when a new item is added, but leave it.
-Codechange: ShowLastNewsMessage has been changed so that it actually works, wraps
around correctly (array as FIFO) and shows the previous news item if the current
news item is open (previously this was doubly shown).