Not everything can be done with the diveId

In commit c3fe1a9e9f79 ("Get rid of pointers to dive structures in the
UI") I was a bit too aggressive moving away from pointers to dives.

This is only needed for pointers that are held across operations that
could change the dive_table. I figured that it wouldn't hurt to get rid of
some more pointers as well, but it turns out I was wrong. The current dive
that we store in the Cylinder and Weight models can be a dive that isn't
in the dive_table at all: the multiEditEquipmentPlaceholder. And when
using the diveId we end up finding the original dive in the dive_table and
therefore modify the wrong structure.

This undoes two thirds of the above mentioned commit.

Reported-and-analyzed-by: Patrick Valsecchi <patrick@thus.ch>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>