Move Appdata file to /usr/share/metainfo

According to the Appstream specifications, Appdata files must now be installed in /usr/share/metainfo. /usr/share/appdata is a legacy path.

See: https://www.freedesktop.org/software/appstream/docs/sect-Metadata-Application.html and: https://www.freedesktop.org/software/appstream/docs/chap-Metadata.html