[language] add GUI setting descriptions

As discussed here http://forum.xbmc.org/showthread.php?tid=164306&pid=1416637
99% is c/p from the wiki so any typos where already there(TM).

I've created a string ID for every help="" that was available. It might be not needed yet but there's no harm in it.
A part of it is still empty as I didn't yet look into these yet (and some i have no idea what to describe them).

Discussion/suggestions:
http://forum.xbmc.org/showthread.php?tid=164407