Fix monster group debug

#8397 broke the monster group debug menu, also the needs of debugging new-style monster groups are a little different.
Now it displays both hordes and regular monster groups, except forest groups because they're literally everywhere.
It also displays every group on a given tile instead of giving precedence to just the first one.
An interesting result is that swamp spawns are obviously hosed, I'm not sure what happened to them, but now they're easier to debug at least.