Switch to metainfo.xml for AppStream support

[BLumia]

https://appstream.debian.org/sid/main/issues/pineapple-pictures.html

• no-metainfo
  This software component is missing a MetaInfo file to provide metadata about it.

https://freedesktop.org/software/appstream/docs/chap-Metadata.html#spec-component-location

Important: Legacy Path

AppStream tools scan the /usr/share/appdata/ path for legacy compatibility as well. It should not be used anymore by new software though, even on older Linux distributions (like RHEL 7 and Ubuntu 16.04 LTS) the metainfo path is well supported. Support for the legacy path will likely be dropped completely with a future AppStream 1.0 release.

https://docs.fedoraproject.org/en-US/packaging-guidelines/AppData/

appdata.xml files
For historical reasons, AppStream specification also allows using extension .appdata.xml for GUI applications.

https://freedesktop.org/software/appstream/docs/sect-Metadata-Application.html#spec-appdata-introduction

While desktop-application metadata is commonly stored in /usr/share/metainfo/%{id}.metainfo.xml (with a .metainfo.xml extension), using a .appdata.xml extension is also permitted for this component type for legacy compatibility. AppStream implementations will recognize either file type, as long as it ends up in the right location on the filesystem.

• Use .metainfo.xml extension
• Run check to ensure content is valid. appstream-util validate-relax --nonet xxxxxx.metainfo.xml

[BLumia]

Hopefully resolved.

For the record, it seems KDE still generates metainfo file with appdata.xml suffix for plasma applets, seems done in plasma_install_package() but haven't dig-in too much. Under my Archlinux installation, there are still a lot of appdata.xml suffix files under /usr/share/metainfo/ which really makes me feel confused.