BTW: Rename the manufacturer-specific name ``ion-icon`` of the icon set in
SerXNG to ``sxng-icon-set`` --> The icon set can be assembled together as
desired in the FE and should therefore not have a manufacturer-specific name in
the style and template definitions.

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

This patch implements the template generation of the template:

  searx/templates/simple/icons.html

by the way the icon set (the icon names) has been normalized:

  film-outline   --> film
  magnet-outline --> magnet
  ..
  warning        --> alert

Some missing (categorie) icons had been added.

Some of the ionicons are not suitable for a dark theme, we fixed the svg
manually in src/svg/ionicons:

- https://github.com/searxng/searxng/pull/4284#issuecomment-2680550342

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

Seems to me svg2jinja added the class in the past .. but no longer in new builds
/ this patch adds the class back by using addAttributesToSVGElement [1].

To test this patch use:

    $ ./manage themes.simple
    $ cat searx/templates/simple/icons.html

and check `class="ionicon"` is in the outer `<svg ..>` tags.

[1] https://svgo.dev/docs/plugins/add-attributes-to-svg-elements/
[2] https://ionic.io/ionicons

Closes: https://github.com/searxng/searxng/issues/3383
Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>