Use syntax from "enemy" map layer for spawn powers/events

Fixes #1282

This change gives some benefits:
- no crashes, since we've already established a list of valid enemy file
names
- "fuzzy" enemy type selection. For example, we can spawn any type of
antlion by using "spawn_type=antlion"

So to spawn a normal antlion, we used to have:
    spawn_type=enemies/antlion.txt
Now we'll use:
    spawn_type=antlion_normal