QSortFilterProxyModel::sort() always ends up in modelReset() which hits performance badly

Since change-id I16b859d92fdb1823c4a56c297d4451abe438fbb1 there is no difference between layoutChanged() and modelReset() any more. This results in layoutChanged being much slower than before and also impacts operations that used layoutChanged() implicitly.

E.g. emitting dataChanged() on a single item in the model, causes the attached proxy model to call sort(), which in turn ends up calling modelReset(). This means, every time a single item emits dataChanged() the complete ListView delegates are destroyed and recreated. This hits performance really badly and makes it mostly unusable on low end hardware such as mobile phones.