This is a helpful feature (for, e.g. WindowNavigation) that
allows modifiers (if they so choose... the old API remains
supported) to easily send a single Message to the modified
layout in response to a Message.
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.
It seems that GHC 6.6 just enables -fglasgow-exts when it sees any LANGUAGE
pragma, so not all of them were added; this patch adds the rest of them, which
is necessary for xmonad to compile in GHC >=6.7.
