Utilize new display abstractions.
This removes most of the code in the engine which was responsible for creating and stopping X servers. It is still the engine's job to figure out which kind of display to utilize, but the initialization of those displays is delegated to VisItDisplay. This also fixes a small bug and a previously-missing comment in XDisplay.C. git-svn-id: http://visit.ilight.com/svn/visit/trunk/src@5120 18c085ea-50e0-402c-830e-de6fd14e8384