Log Message:
Update to 0.21. Add comment to patch.

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Version 0.21
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 desktop-file-validate
  - update to current version of menu specification (Vincent):
    - only print hint if no main category is present, not an error
    - make Science a main category
    - add new registered categories: Adult, Feed, Humanities, Maps,
      Shooter, Spirituality, XFCE
    - update related categories
    - add TDE to list of registered OnlyShowIn
  - accept and validate GNOME3/GSettings for AutostartCondition (Vincent)
  - output hint if more than one main category is present (Vincent)
  - output hint about suggested related categories (Vincent)

 misc
  - do not require glib >= 2.28 for build (Vincent)