Feature summary:
Display a list of sections present on the Gadgets tab in Preferences.

Use case(s):
When user wants to enable or disable some gadgets or when they want to check what gadgets are available.

Benefits:
There are many gadgets on major wikis. Even though they may be organized into sections, there's no way to navigate to a particular section (apart from anchor in the URL). Furthermore, in order to know, what types of gadgets are offered one has to browse through the whole list (as opposed to taking a glimpse on a ToC at the top of the list and jumping right to the relevant section).

Currently, a substitute of a real ToC can be hard-typed into (gadgets-prefstext) message, albeit it will not follow hiding sections where all gadgets are hidden due to e.g. insufficient user permissions.