Keep current item and selection when resorting, part 1

Keep current item and selection when resorting, part 1

KFileItemModel now emits the itemsMoved signal when the model
is resorted, and KItemListSelectionManager has a new function
itemsMoved() which will be called indirectly when this signal
is emitted. Unit tests for the new functionality are included.
The following things are still needed to make the feature
work:
1. KFileItemMdel::resortAllItems() should not emit
   itemsAdded/itemsRemoved any more.
2. KItemListView::itemsMoved() must update the view according
   to the changes in the model, and it must call
   KItemListSelectionManager::itemsMoved().