Add a way to update the modifier in X.L.LayoutModifier
Add a way to update the modifier in X.L.LayoutModifier

Ignore-this: f257a376bef57689287b68ed21ec903d

This patch adds the possibility to update the state of a layout modifier when
modifying the underlying layout before it is run(i.e. using modifyLayout).
The modified state is also passed to the subsequent call of redoLayout, whose
return takes precedence if both functions return modified states of the layout
modifier.

darcs-hash:20090822213958-7f603-2116263ba9622458aa9dea0f664af4431775a361

kmels/xmonad-launcher,kmels/xmonad-launcher