Skip some unused compiles in XEH

[commy2]

• don't compile events handlers except preStart when starting the game. They are discarded anyway, so this needlesly slows down starting the game.
• don't compile preStart event handlers in missions. They are never used while the game is running.

[PabstMirror]

Yes!, I had been thinking of doing something like this. Faster at pre-start, faster at mission load.