Mobile: move tripId from DiveObjectHelper to DiveListModel

The canonical way of displaying lists in Qt is via models.
Thus, return the tripId directly from the DiveListModel instead
of going indirectly via a DiveObjectHelper. In the future, this
will allow us to make the DiveObjectHelper value-based, as it
is not generated numerous times for every list item.

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

qt-models/divelistmodel.h

@@ -38,6 +38,7 @@ public:
 	enum DiveListRoles {
 		DiveRole = Qt::UserRole + 1,
 		DiveDateRole,
+		TripIdRole,
 	};
 
 	static DiveListModel *instance();