Better handling of multiple versions in the search

[lazka]

Long term solution:

• Provide an interface to include/exclude specific modules from the search (defaults to including only the newest version for each module). Safe the state in a cookie.

Short term solution:

• Only include the newest version in the search.

[lazka]

We now have a config page: https://lazka.github.io/pgi-docs/#config.html

• Add a "Search all modules" checkbox which is checked by default.
• Add a list of checkboxes of all included modules sorted by name. Provide buttons for "Select All" "Unselect All"

Provide a button "Restore defaults" which resets the whole config page.

[lazka]

For the search index: Move "objects" into a "namespace" dict , leave everything else as is. {"Gtk-3.0": "objects" : {...}}