Rename debug category for consistency, and add it to categories file.
Summary: ... and disable it by default, like the others.
Test Plan: kdebugsettings shows it, and kmail is more silent by default.
Reviewers: mlaurent, dvratil
Reviewed By: dvratil
Subscribers: KDE PIM
Tags: KDE PIM
Differential Revision: https://phabricator.kde.org/D8778