Move addScript, addStyle and addHeader from OC_Util to OC_TemplateLayout

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

we better use the new functions which are available now: https://github.com/owncloud/core/commit/7e4d24420096404e5902e66ff568d0a01f8a662e