Summary:
This was a regression caused by the code attempting to insert new items
at drop position, if available. `setSortMode` was being called in a slot
connected to the dir model's rowsInserted, but the Positioner has to be
initialized earlier as a proxy needs to handle
`sourceRowsAboutToBeInserted` as well.

Thanks to an investigation and patch by Oleg Solovyov in D17689 for
helping to get to the bottom of this.

This is aimed at 5.12+.

BUG:401023

Reviewers: #plasma, McPain, davidedmundson

Reviewed By: #plasma, davidedmundson

Subscribers: ngraham, davidedmundson, fvogt, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18182