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>

desktop-widgets/command_edit_trip.cpp

@@ -0,0 +1,85 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include "command_edit_trip.h"
+#include "command_private.h"
+#include "core/divelist.h" // for mark_divelist_changed(). TODO: remove
+#include "core/qthelper.h"
+
+namespace Command {
+
+EditTripBase::EditTripBase(dive_trip *tripIn, const QString &newValue) : trip(tripIn),
+	value(newValue)
+{
+}
+
+// Note: Virtual functions cannot be called in the constructor.
+// Therefore, setting of the title is done here.
+bool EditTripBase::workToBeDone()
+{
+	setText(tr("Edit %1").arg(fieldName()));
+	return data(trip) != value;
+}
+
+void EditTripBase::undo()
+{
+	QString old = data(trip);
+	set(trip, value);
+	value = old;
+
+	emit diveListNotifier.tripChanged(trip, fieldId());
+	mark_divelist_changed(true);
+}
+
+// Undo and redo do the same as just the stored value is exchanged
+void EditTripBase::redo()
+{
+	undo();
+}
+
+// Implementation of virtual functions
+
+// ***** Location *****
+void EditTripLocation::set(dive_trip *t, const QString &s) const
+{
+	free(t->location);
+	t->location = copy_qstring(s);
+}
+
+QString EditTripLocation::data(dive_trip *t) const
+{
+	return QString(t->location);
+}
+
+QString EditTripLocation::fieldName() const
+{
+	return tr("trip location");
+}
+
+TripField EditTripLocation::fieldId() const
+{
+	return TripField::LOCATION;
+}
+
+// ***** Notes *****
+void EditTripNotes::set(dive_trip *t, const QString &s) const
+{
+	free(t->notes);
+	t->notes = copy_qstring(s);
+}
+
+QString EditTripNotes::data(dive_trip *t) const
+{
+	return QString(t->notes);
+}
+
+QString EditTripNotes::fieldName() const
+{
+	return tr("trip notes");
+}
+
+TripField EditTripNotes::fieldId() const
+{
+	return TripField::NOTES;
+}
+
+} // namespace Command