Fix reordering (ctrl+dragging) icons to the right

Previously ctrl+dragging an icon towards the right, and dropping it on top of another icon, would leave it 1 icon to the left of the position you dragged it to (to the left of the drop target). This is unintuitive and confusing, inconsistent with dragging an icon to the left, and prevents you from dragging a launcher to the rightmost position in the dock. This fixes it.

I'm not sure why the --destpos; was added in the first place, in 81b55de0 (over 2 years ago). Is there now a risk of dropping an icon at position "length", which is an invalid position? I don't think so. In my testing, if I tried to drop the icon to the right of the rightmost icon/group (which is a Separator, not part of Docklike), the drag would be cancelled and the icon wouldn't be moved (rather than being moved out-of-bounds).