[refactor] icon engines: add new icon category (#4817)

Icons category makes sense because it allows to quickly search for free SVG
icons to use for websites / other designs with a quick `!icons` query

Icons don't seem to fit into the normal images category that well because icons
are quite a special type of images

searx/settings.yml

@@ -1053,7 +1053,6 @@ engines:
 
   - name: material icons
     engine: material_icons
-    categories: images
     shortcut: mi
     disabled: true