Minor UI adjustment in the settings menu

Update
- Added a slider so leveling any other skill now also levels the intellectual skill (default 5%). So assuming you are gaining 10 XP for Plants, then 0.5 XP are also learned for Intellectual). Turn to zero if you dont want this effect at all.

Update:
- Added two new sliders to the settings menu that allow you to control how quickly XP for the intellectual skill is gained and lost (decay).
- Response to player feedback who wanted to be able to "freeze" the intellectual skill where it is (or to at least slow the learning down)

[Auto-generated text]: Initial upload.