Right clicking a job preference level will now lower it.

It always made me sigh when I wanted to lower a job preference but I needed to click it all the way through - so I decided to implement a right click option. Left clicking the preference (like [Medium]) will raise it like it did before, right clicking will lower it.
I think I also cleaned the code a bit, I had hard times understanding how it works.

First pull request/modification to BYOND code, yey?