Refactored some code

- Refactored the logic to determine the slot integer out of the
CheckHeadpiece() function into own method, retuning an integer and
therefore removing the useless slot field.
- Removed KeyPressed() method and replaced it with
HeadpieceToggleKeyPressed()
- Added update logic to oosPlayerUtilities.psc
Fixed bug with immersive Armours v6

Fixed a bug where the hoods from immersive armours version 6 would not
be hidden correctly.
This fix needs to be tested with IA v7