in Gimp, you can change the title of image windows (in File -
Preferences - Interface - Image Windows - Title & Status).
But I could not find any help about the meaning of the letters which
are used in combination with %.
Is there a complete list to be found somewhere?
Yes. It's in the gimprc manpage, also available online:
Thanks. Also to Yosh.
Not a very obvious place to look for it ...
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