cylinders: only hide cylinders at the end of the list

On the equipment tab, unused cylinders (automatically added,
no pressure data) could be hidden. This was implemented using
a QSortFilterProxyModel.

Apparently, it causes confusion if cylinders in the middle of
the list are hidden. Therefore, only hide cylinders at the end
of the list.

QSortFilterProxyModel seems the wrong tool for that job, so
remove it and add a flag "hideUnused" to the base model. Calculate
the number of cylinders when changing the dive.

This is rather complex, because the same model is used for
the planner (which doesn't hide cylinders) and the equipment
tab (which does). Of course, syncing core and model now becomes
harder. For instance, the caching of the number of rows was removed
in a37939889b and now has to be
readded.

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

qt-models/diveplannermodel.cpp

@@ -446,7 +446,7 @@ int DivePlannerPointsModel::rowCount(const QModelIndex&) const
 
 DivePlannerPointsModel::DivePlannerPointsModel(QObject *parent) : QAbstractTableModel(parent),
 	d(nullptr),
-	cylinders(true),
+	cylinders(true, false),
 	mode(NOTHING)
 {
 	memset(&diveplan, 0, sizeof(diveplan));