[Xmonad] patch: give WindowNavigation an empty description

Hi all, The WindowNavigation layout modifier was using the default "show" as its description, leading to a lengthy mess when displaying the current layout (using e.g. DynamicLog). This patch changes the description to be empty, the reasoning being that I know whether I'm using WindowNavigation or not and don't need to see it in front of the layout. I've also included a small change to LayoutModifier so that an extra space is not added before the layout description when the modifier description is empty. -Brent