Defer initial positions apply until listing is complete

Summary:
This fixes the infamous "desktop positions partially scramble on reboot"
bug that occurs when KDirLister completes listing in multiple model
transactions.

This also:
* Disallows moves and drops while listing, for extra safety.
* Cleans up wonky old defer-sometimes code that made little sense.
* Removes a cache for lastRow() that was never actually used.

BUG:354802

Reviewers: #plasma, davidedmundson, chinmoyr

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D18598