Change trap detection mechanics

Halved perception's influence on trap detection, and added trapping skill into the calculation. Now you'll have to be both perceptive and skilled at traps to notice traps.

I always disliked how you absolutely had to have at least 10 perception if you wanted to be able to detect landmines. Now you can make up for lower perception with more trapping skill; you can now detect landmines at 10 per and 5 trapping skill, 8 per and 6 trapping skill, 6 per and 7 trapping skill, etc.