Add new config value, defaultMenuGap, for specifying a gap for a status bar

By default, it is 0 (set in Config.hs), but set this to a pixel count to
get a permanent gap at the top of the screen. You can then at startup
launch dzen, and it will run in this gap, and not be obscured by other
windows.

Perfect for a persistant status bar.

Config.hs-boot

@@ -1,3 +1,4 @@
 module Config where
 import Graphics.X11.Xlib.Types (Dimension)
 borderWidth :: Dimension
+defaultMenuGap :: Int