Add OARS tags to the appdata
Summary: Adds OARS tags to the appdata, they're used to give ratings in App Stores like KDE Discover https://hughsie.github.io/oars/
Reviewers: apol, aacid, KDE Games, ltoscano
Reviewed By: ltoscano
Subscribers: ltoscano, KDE Games
Differential Revision: https://phabricator.kde.org/D12154