[kcoredirlister] Remove as many url.toString() as possible

Summary:
Change itemsInUse, itemsCached and DirectoryDataHash keys from QString to QUrl, avoiding as many QUrl toString() as possible.

Test Plan: It passes the autotests using 20 msecs less.

Reviewers: dfaure, Frameworks

Reviewed By: dfaure

Subscribers: apol, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D13189