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