Use the KFileItemListProperties copy constructor instead of the conversion from KFileItemList to KFileItemListProperties. This saves re-evaluating all items.

Summary: As the title says. Minor optimization.

Test Plan:
Ran the testcases, all pass just fine.
Also in callgrind it's one less call to the KFileItemListProperties constructor that takes a KFileItemList. A small win :)

Reviewers: dfaure, jtamate

Reviewed By: jtamate

Subscribers: Frameworks

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D10261