Update_icons() system improvement (Hand layers)

Removes both hand layers and adds a single Hand layer for each mob.
Replaces both update_inv_BLAH_hand() procs with update_inv_hands()

The affected mobs are as follows:
* Humans - [Tested]
* Monkeys - [Tested]
* Xenomorphs - [Tested]

This puts us a little bit closer to mobs having more than 2 hands!