Reorder for Z-level ops

With Reorder off, each Z level is roughed completely before moving to a lower depth.

With Reorder on, paths are created that machine vertical regions as shown below.

Parent topic: Milling tab (Z-level rough)
Parent topic: Milling tab (Z-level finish)
Parent topic: Milling tab (2D spiral finish)
Parent topic: Milling tab (Horizontal + vertical strategy) - 2nd (steep) operation