Fix sorting-issues when value of a sort-role has been changed

Fix sorting-issues when value of a sort-role has been changed

If the value of a sort-role has been changed, emitting the signal
itemsChanged() is not sufficient as from the KItemModelBase point
of view an item has been moved or deleted/reinserted.

Corresponding to the unit-test KFileItemModel::setData() respects
this case correctly now, however there are some minor visual
animation issues left that (hopefully) should not be tricky to
solve.