sortproxymodel: check if source model is a proxy model

if the source model is a proxy model, the source model's source
model might be changed and our sortproxymodel won't get notified.
This might happen when we try to pass a QML-based proxy model as
source model, like KItemModel.KSortFilterProxyModel for example.

qt/model_view/sortProxyModel/sortproxymodel.cpp

@@ -133,6 +133,11 @@ void SortProxyModel::setSourceModel(QAbstractItemModel *model)
             connect(model, &QAbstractItemModel::dataChanged, this, &SortProxyModel::handleDataChanged);
             connect(model, &QAbstractItemModel::rowsInserted, this, &SortProxyModel::handleRowsInserted);
             connect(model, &QAbstractItemModel::rowsRemoved, this, &SortProxyModel::handleRowsRemoved);
+            QAbstractProxyModel * proxyModel = qobject_cast<QAbstractProxyModel*>(model);
+            if (proxyModel)
+            {
+                connect(proxyModel, &QAbstractProxyModel::sourceModelChanged, this, &SortProxyModel::resetInternalData);
+            }
         }
         endResetModel();
     }