Add a customheader.tpl and customfooter.tpl in skins
The goal is to simplify the skin customization. For now, we can link to HTML templates and then modify CSS and pictures. But we can maybe include by default a custom header and custom footer that will not contain any logical code, so everyone will ba able to put its own HTML code into it.
We should also let the possibility to change the favicon, and maybe include some js and other CSS files -> use a customhtmlhead.tpl ?