Add a shadow to symbols in frame buttons.

themes/default/close-active.xpm

themes/default/close-pressed.xpm

themes/default/hide-active.xpm

themes/default/hide-pressed.xpm

themes/default/maximize-active.xpm

themes/default/maximize-pressed.xpm

themes/default/maximize-toggled-active.xpm

themes/default/maximize-toggled-pressed.xpm

themes/default/shade-active.xpm

themes/default/shade-pressed.xpm

themes/default/shade-toggled-active.xpm

themes/default/shade-toggled-pressed.xpm

themes/default/stick-active.xpm

themes/default/stick-pressed.xpm

themes/default/stick-toggled-active.xpm

themes/default/stick-toggled-pressed.xpm