run missions preInit XEH on mission switch in 3DEN

[commy2]

When merged this pull request will:

• runs the missions XEH preInit eventhandlers every time a new mission is selected in the Eden-Editor ("OnMissionNew" and "OnMissionLoad")
• moves "OnTerrainNew" fix to the 3DEN display load event, so the 3DEN eventhandler isn't repeatedly added by itself every time the terrain is switched in the Eden-Editor.
• also renames 3DENDisplayLoad to initDisplay3DEN ("init" of "Display3DEN")

This isn't a perfect fix, because functions or settings defined in the missions preInit will still carry over to the new mission / loaded mission if the terrain was not switched. This fixes itself when the preview is used though. Currently using the preview would delete the carried over functions and settings and also initialize the new ones, so at least half the problem is gone.