[feat] categories: add icons for remaining category tabs (#6759)

Icons are provided for the main categories known to us.

The distinction between CATEGORY_GROUPS and CATEGORY_NAMES is abandoned in favor
of the latter.

HINT: we need single-color icons; transparent background and the color must be
`currentColor` so that the icons can be used in light/dark themes.

The following SVGs from the ionicons had to be revised (use ``currentColor``).

- client/simple/src/svg/ionicons/logo-rss.svg
- client/simple/src/svg/ionicons/radio-outline.svg
- client/simple/src/svg/ionicons/color-palette-outline.svg

For the following categories, I did not find any usable icons in Ionicons; new
SVGs were created for these categories.

- onions: client/simple/src/svg/ionicons/tor-browser.svg
- lyrics: client/simple/src/svg/ionicons/song-lyric-music.svg [1]
- packages: client/simple/src/svg/ionicons/package_2.svg [1]

[1] Based on Google's Material icons: https://fonts.google.com/icons

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

client/simple/tools/jinja_svg_catalog.html.edge

@@ -7,9 +7,18 @@ for the backend.  Example output of this EDGE template:
 --}}
 {#
 Catalog of SVG symbols that can be inserted into the HTML output of a Jinja
-template. This file from:
+template. This file is generated by:
+
+    make static.build.commit
+
+from template:
+
+    client/simple/tools/jinja_svg_catalog.html.edge
+
+To modify the icon set, edit the script:
+
+    client/simple/theme_icons.ts
 
-  client/simple/tools/icon_catalog.edge.html
 #}
 
 {%-