add a formfactor session variable which is autodetected but can also manually...

add a formfactor session variable which is autodetected but can also manually overwritten via a get variable.
currently we have:
         * default -> the normal desktop browser interface
         * mobile -> interface for smartphones
         * tablet -> interface for tablets
         * standalone -> the default interface but without header, footer and sidebar. just the application. useful to use just a specific app on the desktop in a standalone window.
In the future we should adapt the userinterface to the specific formfactor.