Undo: implement undo of dive trip editing

This is copying the dive editing code. It uses an OO design with
virtual functions for getting and setting the values. It doesn't
use templates though, as both fields of strig type. This feels
a bit over-engineered, but it is 1) consistent with the dive edit
code and 2) the number / types of dive trip fields might increase.

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

core/subsurface-qt/DiveListNotifier.h

@@ -10,7 +10,7 @@
 
 #include <QObject>
 
-// Dive fields that can be edited.
+// Dive and trip fields that can be edited.
 // Use "enum class" to not polute the global name space.
 enum class DiveField {
 	NR,
@@ -29,6 +29,10 @@ enum class DiveField {
 	MODE,
 	NOTES,
 };
+enum class TripField {
+	LOCATION,
+	NOTES
+};
 
 class DiveListNotifier : public QObject {
 	Q_OBJECT
@@ -54,6 +58,9 @@ signals:
 	void cylindersReset(dive_trip *trip, const QVector<dive *> &dives);
 	void weightsystemsReset(dive_trip *trip, const QVector<dive *> &dives);
 
+	// Trip edited signal
+	void tripChanged(dive_trip *trip, TripField field);
+
 	// Selection-signals come in two kinds:
 	//  - divesSelected, divesDeselected and currentDiveChanged are finer grained and are
 	//    called batch-wise per trip (except currentDiveChanged, of course). These signals