[enh] change categories_as_tabs from a list to a dict

The tab icon names are currently hard coded in the templates.
This commit lets us introduce an icon property in the future, e.g:

categories_as_tabs:
  general:
    icon: search-outline

tests/robot/settings_robot.yml

@@ -34,8 +34,8 @@ outgoing:
   useragent_suffix: ""
 
 categories_as_tabs:
-  - general
-  - dummy
+  general:
+  dummy:
 
 engines:
   - name: general dummy