[Tags] Lower default log level to warning, add to categories file

Summary:
The tags: ioslave had the old default log level of "all", switch of
Debug messages by default.
Add to categories file to allow altering with kdebugsettings.

Test Plan:
Watch the journal
Enter URL tags:/<some tag>
-> journal is no longer filled with Debug messages

Reviewers: Baloo, Frameworks, poboiko, ngraham

Reviewed By: Baloo, ngraham

Subscribers: ngraham, kde-frameworks-devel

Tags: Frameworks, Baloo

Differential Revision: https://phabricator.kde.org/D16168