Instead of a "Cylinder for planning" use a default cylinder

Right now hardcoded to AL80. This way in the future we'll have a volume of
gas that's available. And this makes much more sense then a random string
in the description field.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Connect changes in the tanks with the dive that is being added

- you can no longer delete a tank when its gas is in use
- therefore you can no longer delete the last tank
- when you change the gas mix of a tank, the corresponding segments in the
  dive change as well
- when changing gas for a segment the correct available gases are offered

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Don't change gas used in segments if there is still a cylinder with that gas

Otherwise adding a second cylinder (to the default one cylinder of air)
and subsequently changing the gas of that cylinder will illogically change
the gas used in all the segments.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Correct behavior when adding a dive to an empty list and canceling the add

We need to leave the widget disabled and the date / time and coordinates
cleared.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Don't overwrite the new cylinders when adding a dive

We carefully assembled the correct data in the cylinders - don't replace
that data when manually editing a dive.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Fix addGas to look at the correct cylinder

That was a stupid bug. You have to actually update the variable to look at
the right cylinder. Duh.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Correctly convert an existing dive into a plan

I missed this one spot when converting back to the different semantics for
divedatapoints.

With this change add dive appears to work correctly (fingers crossed).

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