Separate users into "normal" and "advanced"

XWiki offers many features, and this might seem overwhelming for beginners. We should add an option in the user settings, allowing them to select the normal or advanced interface.

Advanced interface is the one we have now. The normal interface will hide many options from the menu.

In an ideal wiki, administrators develop applications, i.e. create classes, sheets, templates, panels, and normal users just click on a link and fill-in a web form. This way class and object editors are not needed, the default editor should be the only one needed, code, xml and history are not user's concerns, etc.