File names that have lots of spaces look quite ugly when inserted as
completions because every space will have a backslash.
Add an initial heuristic to decide when to use quotes instead of
backslash escapes.
Quote when
1. it's not an autosuggestion
2. we replace the token or insert a fresh one
3. we will add a space at the end
In future we could relax some of these requirements.
Requirement 2 means we don't quote when appending to an existing token.
Need to find a natural behavior here.
Re 3, if the completion adds no space, users will probably want to add more
characters, which looks a bit weird if the token has a trailing quote.
We could relax this requirement for directory completions, so «ls so»
completes to «ls 'some dir with spaces'/».
Closes#5433
