This patch is another fix to the positioning of icons on the
pinboard. When an icon is dropped into the pinboard for the first
time, the position of the dropping cursor (which, while dropping,
appears as the top-left corner of the icon being dropped) is used as
the center position for the new icon, which causes a disconcerting
jump in the icon's position. This patch fixes that, making the icon
stay where it gets dropped.