Add a key to have nothing in hands

Instead of leaving an empty slot in the Weapon Hot Bar to have nothing in hands, let us configure a key to take off weapons.

Some games handle this by letting you press the key of an equipped item to unequip it. Like, say I have a confetti gun in slot 1, I just press key 1 to pull it out, and then key 1 again to put it away, and so on for the entire hotbar.