Desktop: Fix Gas Editing for Manually Added Dives.

- show the correct gasmix in the profile;
- make gases available for gas switches in the profile after they have
  been added;
- persist gas changes;
- add air as a default gas when adding a dive.

This still has problems when undoing a gas switch - instead of
completely removing the gas switch it is just moved to the next point in the
profile.

Signed-off-by: Michael Keller <github@ike.ch>

qt-models/diveplannermodel.h

@@ -23,12 +23,12 @@ public:
 		GAS,
 		CCSETPOINT,
 		DIVEMODE,
-		COLUMNS
+		COLUMNS,
 	};
 	enum Mode {
 		NOTHING,
 		PLAN,
-		ADD
+		EDIT,
 	};
 	int columnCount(const QModelIndex &parent = QModelIndex()) const override;
 	QVariant data(const QModelIndex &index, int role = Qt::DisplayRole) const override;