Motivation
Right now, AdvancedSearch has a predefined set of keywords in the "advanced parameters" list, but there is no way for users to include other ones. However, this would often be helpful, e.g. to add all Wikidata unique keywords to the Wikidata search.

Acceptance Criteria
[] Add a hook to AdvancedSearch that allows gadgets or other extensions to add keywords to the advancedParameters panel. Modifying existing sections is not part of this ticket.

• All new keywords should appear below the existing keywords in a new section
• When new keywords were added through a hook, the search query would include both keyword and user input for the keyword, and return the respective results
• The general look of AdvancedSearch should not change - for anyone not adding keywords with the hook, it should stay exactly as it is
• The hook should be documented and example implementations of simple ways of modifying the form should be supplied as part of that documentation.