autocomplete "Autocomplete cf" in search builder

though we already have Elements/EditCustomFieldAutocomplete, it requires a few
code changes to make it work here, 2 main things:

* current ajax custom field values helper requires context object, which we
  don't have one in search builder
* EditCustomFieldAutocomplete is mainly used in objects' edit pages, the input name
  isn't compatible here.

making another simpler version(<10 lines) seems more right here.
search/show Description field too

like we do on cf modify pages
Merge branch '4.0/autocomplete-cf-in-search-builder' into 4.0-trunk