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

(cherry picked from commit 8c51fbac)