[AppStream Runner] Use categorized logging

Summary:
Add category org.kde.plasma.runner.appstream, like done for the Services
runner, i.e. org.kde.plasma.runner.services

Reviewers: #plasma, apol, ngraham

Reviewed By: apol

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D14805