-Codechange: Use the newly created WWT_TEXT widget.
-Cleanup: remove a lot of magic numbers, make the whole window more flexible, with now the possibility to easily add new type of news events.
Thanks glx for ideas from an old patch
-Regression [FS#573] (r8049): ShowLastNewsMessage could show an out-of-bounds news item because it did not checked if a previous item actually existed the first time it is called (forced news is INVALID_NEWS).
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).