Improve appstream

• Rename appstream file to metainfo, as appdata is considered as legacy: https://freedesktop.org/software/appstream/docs/chap-Metadata.html#spec-component-location
• Add "The KDE Community" as developer name as the freedesktop.org documentation uses it as an example: https://freedesktop.org/software/appstream/docs/chap-Metadata.html#tag-developer_name
• Add source code link
• Add offline-only