lib: automatic title: replace fg with description from jobs (#7982)

* Automatic title: Replace fg with description from jobs

* Avoid error messages when there is no job

* Use $jobstates and $jobtexts to look for jobs

`jobs %string` doesn't work correctly when run inside `$()`. `$jobstates` and
`$jobtexts` is available in the current shell process, so even though we need
to replicate a bit more logic, every type of `fg` invocation works correctly.

* lib: clean up termsupport.zsh

Co-authored-by: Marc Cornellà <marc.cornella@live.com>