Encapsulate the horrid gas encoding in gas change events

We should never pass permille values around as integers. And we shouldn't
have to decode the stupid value in more than one place.

This doesn't tackle all the places where we access O2 and He "too early"
and should instead keep passing around a gaxmix. But it's a first step.

Signed-off-by: Dirk Hohndel <dirk@hohndel.org>
Remove the .used member of the cylinder structure

Instead calculate this information on the fly, taking into account all
dive computers on the dive in questions.

There is one wrinkle to this - previously we abused the '.used' member to
make sure that a manually added cylinder didn't disappear the moment it
was added (think of the workflow: you add a cylinder, then you add a gas
change to that cylinder -> right after you add it it is unused and would
not be shown).

I am thinking that we might have to add the "manually_added" property to
the properties that we store in XML / git.

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