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>

desktop-widgets/tab-widgets/TabDiveInformation.h

@@ -23,6 +23,7 @@ private slots:
 private:
 	Ui::TabDiveInformation *ui;
 	void updateProfile();
+	void updateSalinity();
 	void updateWhen();
 	int pressTypeIndex;
 	void updateTextBox(int event);