Take only used gases into account when showing gas in divelist

Here is a patch that restricts the gases listed in the divelist to those
that are actually used. I seem to have the indentation now under control
but I am not sure about the logic:

1) First gas (with index 0) is always used.

2) If there is a gas switch event, the new gas is also used (determined by
walking the list of dive computers and then the list of events).

Signed-off-by: Robert C. Helling <helling@atdotde.de>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>