added Oasis Conquest

http://resource.openra.net/maps/1470
added Desert Twister by @psydev

http://resource.openra.net/maps/990
added Vladimir's Folly

http://resource.openra.net/maps/1639/
Merge pull request #6844 from Mailaender/new-d2k-maps

Added new maps to the Dune 2000 mod