default to empty description for layout modifiers.
default to empty description for layout modifiers.

This is because modifierDescription is designed to be human-readable,
and show rarely creates a human-readable description.  And in many (if
not most) cases, an empty description is precisely what we want.

darcs-hash:20071018202604-72aca-6da6f02e804c95da4cbccae205f83217333f7c96

yzhou61/xmonad-contrib,yzhou61/xmonad-contrib,yzhou61/xmonad-contrib