WIP: Guided config for c++

This adds some templates to the gadget config, organised by "category"
(human-readable) and presented in a menu. The configuration thus created
is just run normally through the variable replacements.

Also:
- Fix default value replacements which come from the calculus
- Add default-value _lists_. This uses select-from-list UI. Useful for
  enumerated values. Allow specifying a default.

autoload/vimspector.vim

@@ -296,7 +296,7 @@ function! vimspector#GetConfigurations() abort
     return
   endif
   let configurations = py3eval(
-        \ 'list( _vimspector_session.GetConfigurations( {} )[ 1 ].keys() )'
+        \ 'list( _vimspector_session.GetConfigurations().keys() )'
         \ . ' if _vimspector_session else []' )
   return configurations
 endfunction