[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