Fix an edge case of divider-obsoletion in Group.

Fix an edge case of divider-obsoletion in Group.

When you drag group elements around, previous dividers are meant to
dissolve whenever the same two elements are no longer on each side of
it. One case in which this didn't happen was that of dragging an
element from the left of a divider to the far right column - the
divider became invisible, but would then startlingly reappear if you
drag that element back to the left of whatever it was left of before.

[originally from svn r10051]