Add option to call a scenario special node to apply the effect of a trap

Also:
- The possible traps resulting from "random trap" is expanded to include the knife trap
- A custom picture can be specified for the trap dialog
- A given custom trap special node can be reused for the same trap with different levels - the pointer -5 holds the trap level passed to the ONCE_TRAP node

rsrc/strings/trap-types.txt

@@ -9,4 +9,5 @@ Alarm (town hostile)
 Flames
 Dumbfound
 Disease
-Foul Spray (disease all)
+Foul Spray (disease all)
+Custom Effect