cubicweb #2439846 Add autocompletion on RQL search bar [done]

It would be nice to provide suggestions while a user types some RQL in the search bar.

prioritynormal
typeenhancement
done in3.16.0
load0.750
load left0.000
closed by#dcd9bc1d1bca [ui] provide an autocomplete RQL bar (closes #2439846), #e2913c9880a0 [test rql completion] dont limit number of completion, else we've impredicable results. Also should tell rql.parse to not print error on stdout