[Qt5-feedback] Add internalVariant to QModelIndex

[Nils Jeisecke]

> I don't know what assert you mean here, and I know of nothing in the code
> preventing you from creating two QModelIndexes with different internal
> pointers and referring to the same item. I also know that you can create
> QModelIndexes dynamically and you don't have to keep them around.

see QAbstractItemView::setModel(QAbstractItemModel *model)

    // These asserts do basic sanity checking of the model
    Q_ASSERT_X(d->model->index(0,0) == d->model->index(0,0),
               "QAbstractItemView::setModel",
               "A model should return the exact same index "
               "(including its internal id/pointer) when asked for it
twice in a row.");

Nils