Added GetItemNameExtra and fixed documentation

Item:
Documentation listed function "Get/SetItemExtraText" whereas the
function was actually called "SetItemNameExtra", and the "Get" version
didn't even exist!

Added "GetItemNameExtra" for returning the sub-name of an item.

Button:
Not that it would really make a difference, but moved the player
pos/world/interior setting to below the "Streamer_UpdateEx" call.

Modules/Dispenser:
Some undefined colour code corrected to simply "0xFFFF00FF" (yellow)