bug#43687: Web search should be customizable

[Jean Louis]

In free software movement, we try to avoid non-free Javascript
websites, so in my opinion, the web search should be customizable:

> Amazon Bing Dictionary Elisp Google Hub(git) Images Maps RFCs
>  StackOverflow Twitter Wikipedia Youtube F-Droid DuckDuckGo

Examples how I would personally customize it:

- I would myself put my self-hosted searx instance at first place, so
  I would like to easily customize Hyperbole web search

- Amazon and Bing I would remove for privacy and Javascript reasons

- Hub(git) I would rename to call it Github as original

- Images is unclear, yet it uses Google, I would modify it to use
  Duckduckgo or other privacy search engine

- Maps uses Google, I would use Openstreetmaps or other privacy search

- I would remove Twitter and use GNU Social or Pleroma searches

- I would remove YouTube and use other video search

In general customization should contain:

- Name of the search engine

- the URL with some replacements for the search query, such as %s

- the letter that should be upcased or its key press

In general, Hyperbole should not hardcode various external resources.

Jean



I use:  Editor:      GNU Emacs 28.0.50 (build 15, x86_64-pc-linux-gnu, X 
toolkit, cairo version 1.14.8, Xaw3d scroll bars)

        Hyperbole:   7.1.2
        Sys Type:    x86_64-pc-linux-gnu
        OS Type:     gnu/linux
        Window Sys:  x
        News Reader: Gnus v5.13