feat(feh): deassociate with xcf, pdf, dvi, txt

[Maelan]

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[akinomyoga]

- opening *.xcf (Gimp vectorial image) is mentioned in feh’s manpage
  but I have been unable to open any such file;

What is the problem in this case? According to the man page, it seems to use ImageMagick for .xcf when it is enabled by --conversion-timeout. There are many possibilities:

• ImageMagick stopped supporting .xcf after the corresponding part of the man page of feh was written. In this case, we can drop .xcf from the list.
• ImageMagick needs a special configuration to open .xcf. In this case, some users may set it up, so I'm not sure if we should simply drop it.
• ImageMagick works, but feh cannot use the result of ImageMagick for some reason. In this case, maybe we should ask the upstream feh about whether this is intentional.

- opening *.ps, *.pdf, *.dvi (document files) works through ImageMagick,
  but only when the document contains a single page;
  + PDF and DVI are primarily used for possibly-multipage textual
    documents, PDF viewers are widely available and they are much better
    for this purpose than an image viewer that first converts documents
    to bitmaps; it’s unlikely that a user wants to use feh for these.

Is it? The images that will be embedded in PDFs are usually generated as single-page PDFs. I always create plots in academic papers as single-page PDFs. Although I'm not a user of feh, I'm not sure if we can easily conclude that no users use feh to preview images in the form of single-page PDFs.

[scop]

I have not tested the behavior, but I'm inclined to agree with @akinomyoga here, unless some more feedback confirming that deassociating these would be the right thing to do.

[scop]

Closing due to lack of feedback.