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

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.


MP2E/XMonadContrib,MP2E/XMonadContrib