Set up the dive that was passed in as first dive

We carefully extracted the first dive passed in with the QList, but then
always used current_dive. That's silly.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Clean up the setDive function

* This should never be called with a NULL dive.
* The dc variable was set then never used.
* We now have a unique id for each dive, but we can't simply not
  repopulate the model in setDive as the old comment might make you
  consider, as otherwise the dataModel will reference already freed data
  in pInfo.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Don't repopulate the model if the dive didn't change

plotDives takes a list of dives (for future use) but currently only looks
at the first dive in this list. With that semantic in mind we can save
ourselves some work if we first check if this is the same dive we are
already showing.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>