allow layout modifiers to modify a Message in transit.
allow layout modifiers to modify a Message in transit.

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.

darcs-hash:20071020191542-72aca-fd6b8f2edb91519b4e475319c2bc8c3d4b77136d

kmels/xmonad-launcher,kmels/xmonad-launcher