The first gaschange event is hidden as it is considered unimportant.

However in certain scenarios Subsurface imports "phantom cylinders" which did not actually exist on a dive. If such a "phantom cylinder" is attached to the first gaschange event, there is no way for the user to delete such a cylinder, since there is no way to edit or delete the first gaschange event using the UI.

This change adds an option to show the first gaschange event. Or rather, to hide it, and it defaults to true. This means that by default, the program behaves as it always did, but now users have an option to toggle hiding this event. If it s shown, the user can edit or delete the event, and thus edit or delete the cylinder to which the event is attached.

Signed-off-by: Micha WERLE <micha@michaelwerle.com>

When the `Map Short Names` feature was implemented, it was forgotten to
explicitly initialize the preference setting. Implicitly it defaulted to
false, however, it's better to perform this initialization explicitly.

Signed-off-by: Micha WERLE <micha@michaelwerle.com>

Document adding the `hide first gas change` option to the change log.

Signed-off-by: Micha WERLE <micha@michaelwerle.com>