Mobile: move tripNrDive from DiveObjectHelper to DiveListModel

We don't want to generate a DiveObjectHelper numerous times for
every item in the dive list. Therefore, return this datum directly
from the model.

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

qt-models/divelistmodel.h

@@ -39,6 +39,7 @@ public:
 		DiveRole = Qt::UserRole + 1,
 		DiveDateRole,
 		TripIdRole,
+		TripNrDivesRole,
 	};
 
 	static DiveListModel *instance();