use QLoggingCategory for logging

Summary:
Logging with QLoggingCategory instead of QtGlobal functions (qDebug, qInfo, etc)
allow users to filter out unwanted messages with kdebugsettings and qtlogging.ini

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: mlaurent, kde-pim

Tags: #kde_pim

Differential Revision: https://phabricator.kde.org/D26246