Fix possible crash if a kioslave adds multiple items with the same URL

When opening the URL "man:", there are multiple items with the same
name (for example, _exit is shown twice here). When opening a new tab,
the kioslave reports some items as deleted (I have not quite understood
why). The problem is that it reports some of the duplicate items twice
in the list of deleted items. This confused KFileItemModel and
corrupted the internal data structures, and finally, caused a crash.

The fix is to remove all duplicates from
KItemRangeList::fromSortedContainer(const Container& container).

New unit tests included.


REVIEW: 118507
FIXED-IN: 4.13.2