Hints for nikwi in xenial

Errors

• icon-format-unsupported
  Icon file 'nikwi.xpm' uses an unsupported image file format.
• gui-app-without-icon
  The GUI application (application which has a .desktop file for the XDG menu and Type=Application) with AppStream-ID 'nikwi.desktop' has been found, but we could not find a matching icon.