[FIX] don't provide autocompletion to m2o fields when a custom operator or filter_domain is provided
[IMP] add separators between all completion sections

Depending on the way the search view is setup, a single item
(e.g. "Search foo for bar") can follow a list of a bunch of
completions. In that case, it is hard to notice that it's not just one
more item of said list.

Add a marker on the first of every completion list in order to catch
1-item lists (or lists without titles/categories) and prepend a small
border every time, so that single-element completions following
lists-with-titles can be noticed.