Textures for empty slots are in the textures/item folder, despite not being actual items

Namely these:

• empty_armor_slot_helmet.png
• empty_armor_slot_chestplate.png
• empty_armor_slot_leggings.png
• empty_armor_slot_boots.png
• empty_armor_slot_shield.png

Despite their close association with items, they themselves are not items and have never been since Indev. As they are not items, having them be placed in the textures/item folder is incorrect and they should be relocated to somewhere in textures/gui instead.