Desktop: make salinity a field known to the undo system

The undo system sets updates individual dive fields on
redo respectively undo. Make salinity such a field, since
it is changed on replanning a dive.

To do this, break out the "update salinity" functionality
into its own function, add an entry to the DiveField enum
and add the corresponding switch-case.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/subsurface-qt/DiveListNotifier.h

@@ -28,6 +28,7 @@ enum class DiveField {
 	TAGS,
 	MODE,
 	NOTES,
+	SALINITY
 };
 enum class TripField {
 	LOCATION,