[xmonad] [patch] narrower BorderResize rectangles placed within border edges

Hi, This patch improves BorderResize's rectangles placement and geometry. I found the current 10px border-size rather too wide to be convenient (eg I often found myself activating resize when trying to copy output from a floating terminal). Also the rectangles are now placed symmetrically within the border and window edges, no longer extend outside on some sides. The width is reduced to 2px which seems to work well enough and still more lenient than the single pixel of many window manager border decorations. Thanks, Jens