KDirModel: port to qCDebug, with its own category

Test Plan:
Builds, and I was able to enable this category in kdebugsettings
and see the debug output from _k_slotNewItems printed out.

Reviewers: mlaurent, sandsmark, ervin

Reviewed By: ervin

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D25249