Added Invisible to store layout state
Added Invisible to store layout state

Invisible is a data type to store information that will be lost when
restarting XMonad (the idea came from David Roundy)

darcs-hash:20070928190107-32816-ce095723ac5deb5cde50eab871416e3edee1e875

kmels/xmonad-launcher,kmels/xmonad-launcher