Add category name as tooltip.

Summary:
This is useful to find out what to write in QT_LOGGING_RULES (e.g. for a
unittest, where the config saved by kdebugsettings won't be loaded).

Test Plan: Moving mouse over category item shows category name (in first tab)

Reviewers: mlaurent

Reviewed By: mlaurent

Subscribers: kde-utils-devel

Differential Revision: https://phabricator.kde.org/D12377