Planner: don't allow negative depths

This patch allows only non-negative depths in the divepoints table as
negative depths confuse the planenr/deco algortihm. The spinbox appearing
in the table should get a setMinimum(0) but I was not able to find the
correct way to implement that.

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>