[501] Refactored the code to remove most icon operations for changing an icon's colour.

I had to manually set all the icons, which are subject to colour change, to be have a white/grey icon, so that the colours added properly, and then have them coloured in the code. This means the icons for runes, the reagent beaker overlay, smoke and etc will all be white, when unchanged. They now all use the new 500+ color variable.

The only noticeable gameplay difference is that you can't choose your graffiti's shade.

**UPDATE**

Removed the icon color blend operations from human's update_icons().
Changed all the hair/facial_hair/eye sprites to use a white base.
Changed the preview screen, in the preference menu, to use ICON_MULTIPLY to be able to use the new white sprites.
Removed the unused and broken cloaking item.
Ninjas now use the alpha variable, removed most of the checks for it.
Updated the documentation a bit.
Added some helper procs for later.