Made the scripted events use plain lua objects instead of memory additive ones.

The correct object copy is already ensured by luabind anyway. And using plain
member (autovars) makes the lua memory being freed more easily.
Added the missing dialogue requested in the second part of the first cave.

Now Kalya will clearly show where the exit is if the monsters haven't been
all beaten.

I also reduced the time the stone sign image is shown.