Hints for gnome-calendar in main

Warnings

• internal-unknown-tag
  The generator emitted a tag 'internal-unknown-tag' which is unknown. This is a bug in the metadata generator, please file a bugreport.
• screenshot-download-error
  Error while downloading screenshot from 'https://git.gnome.org/browse/gnome-calendar/plain/data/appdata/management.png': HTTP request returned status code 404 (Not Found)
  This might be a temporary server issue, or the screenshot is no longer available.