planner: Switch to QSpinBox for SAC

QSpinBox is nice to see how different sac rate would affect our dive
plan.
"Hey, What would happen if I would be stressed and my sac goes up by 50%?"

Signed-off-by: Anton Lundin <glance@acc.umu.se>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>