Move addScript, addStyle and addHeader from OC_Util to OC_TemplateLayout

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

I honestly see no need to move these static stuff from left to right - this is a breaking change and if we agree to walk this path let's do it 'right' and get rid of that static kung fu.

We anyhow better use stuff provided by appframework from my pov