Mobile: Generate DiveObjectHelpers on the fly

Instead of keeping track of a list of DiveObjectHelpers, generate
them on-the-fly in DiveListModel. Thus, there is less danger of
model and core getting out of sync. On the flip-side, now the
DiveListModel and the DiveListSortModel might get out of sync.

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

core/subsurface-qt/DiveObjectHelper.cpp

@@ -67,6 +67,16 @@ DiveObjectHelper::DiveObjectHelper(struct dive *d) :
 {
 }
 
+DiveObjectHelper::operator bool() const
+{
+	return !!m_dive;
+}
+
+bool DiveObjectHelper::operator!() const
+{
+	return !m_dive;
+}
+
 int DiveObjectHelper::number() const
 {
 	return m_dive->number;