Add some depth to the default theme

po/xfwm4.pot

themes/default/bottom-active.xpm

themes/default/bottom-inactive.xpm

themes/default/bottom-left-active.xpm

themes/default/bottom-left-inactive.xpm

themes/default/bottom-right-active.xpm

themes/default/bottom-right-inactive.xpm

themes/default/left-active.xpm

themes/default/left-inactive.xpm

themes/default/right-active.xpm

themes/default/right-inactive.xpm

themes/default/title-1-active.xpm

themes/default/title-2-active.xpm

themes/default/title-3-active.xpm

themes/default/title-4-active.xpm

themes/default/title-5-active.xpm

themes/default/top-left-active.xpm

themes/default/top-left-inactive.xpm

themes/default/top-right-active.xpm

themes/default/top-right-inactive.xpm