Header handling across apps

Current problems with header:
  * height is set to 90 px in apps (eg: https://github.com/georchestra/georchestra/blob/master/mapfishapp/src/main/webapp/app/js/GEOR.js#L257 ) ... one might want a different value.
  * the code is nearly the same in catalogapp, analytics, mapfishapp, extractorapp
  * no easy way to customize it across apps