LP#1730752 Fix minor bug with move-to-last

LP#1730752 Fix minor bug with move-to-last

If you click "Make last visible" on the column that is already the last
visible, it bumps it down one position when it should not move. This
commit adjusts the logic so that columns that are before or already in
the last-visible slot (thus, '<' or '=') will end up in the last-visible
slot, and columns only go after the last-visible slot if they are
already after it.

Signed-off-by: Remington Steed <rjs7@calvin.edu>
Signed-off-by: Bill Erickson <berickxx@gmail.com>