Move addScript, addStyle and addHeader from OC_Util to OC_TemplateLayout

and also some small cleanups.

@icewind1991 @karlitschek @bantu @DeepDiver1975 

I tried to do bigger changes, but after more than a year they are still not accepted. This is a smaller change to remove the dependency between OC_Util and OC_TemplateLayout. I think this change removes some complexity from OC, and sure i can make the change bigger but then the review process will take even longer.