Make KFileItemModel::createMimeData() faster

Moreover, this commit ensures that the order of the URLs in the
QMimeData object is the same as the order of the items in the view.

Selecting many items and copying them to the clipboard could take quite
a bit of time. This is because we used
KDirModel::simplifiedUrlList(urls) to remove child items from the list
of URLs, and this function sorts the URLs internally to make it easier
to find out which of them are child URLs.

However, since commit 5c5d87fe, the
selected indices are already stored in ascending order, and this makes
it easy to detect if an item is a child of the last item that has been
added to the QMimeData.

BUG: 283409
REVIEW: 113515
FIXED-IN: 4.13.0