icons directory is messy

(There is a PR to add a destkop file, but that needs an icon, and we
don't install one.  Looking into that, the icon situation is messy.
This message is limited to cleaning up the icon situation.)

1) We have icons/ with a bunch of contents:

  ./.xvpics/U.32x32x256.xpm
  ./U.16x16x16m.png
  ./U.24x24x16m.png
  ./U.256x256x16m.png
  ./U.32x32x16.bmp
  ./U.32x32x16m.png
  ./U.32x32x256.bmp
  ./U.32x32x256.xpm
  ./U.48x48x16m.png
  ./U.ico
  ./U.svg
  ./Unison.gif
  ./Unison.xpm
  ./success.gif
  ./svg_to_data.ml
  ./unison-icon.ai
  ./unison-logo.gif
  ./unison.haruo.jpg
  ./unison.png
  ./unison.svg

and notably no README.md that explains what these files are for and why.
And, which are compiled versions of others, and why those are checked
in, vs being generated.
At least some them date to "initial import" in 2005.

There is a ml file, but it uses lablgtk2.

There is unison-logo.ai, which is in a proprietary format and I'd like
to remove that.

2) inkscape objects to U.svg as "90 dpi" for older inkscape vs 96 dpi.
Presumably this is for preview images as this is vector.  Does anybody
think any other tools are appropriate?  Any reason not to let inkscape
update and commit?


3) It's normal for programs to install "hicolor" icons, and unison
doesn't.  I suspect we should install some icons, and maybe even with
command line (as syncthing has a desktop file to start a non-gui
daemon).

Opinions about icon installation?

OVERALL) Cleanup would be a good thing independent of anything else.
Absent comments, I may just do a heavy-handed cleanup.