meta: reference correct desktop file in appdata
The AppStream data is automatically completed from a referenced desktop file. The desktop file here, however, is hardcoded to be org.gnome.Contacts. That leads to Software displaying the org.gnome.Contacts icon, even for the .Devel variant.
see: https://www.freedesktop.org/software/appstream/docs/sect-Metadata-Application.html
fixes #289 (closed)