Fix dive planner sidebar depth units

The dive planner always showed the depth in our internal units, ie
millimeter, in the sidebar that showed the plan points.

That made little sense in metric mode, and none at all in imperial. The
_graph_ showed things in meter and feet.

So make the DivePlannerPointsModel always convert things to and from the
user units.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>