[FeatureReqest] auto-delete unneeded backup-files

This FR is related to this Thread on the user-List
<https://groups.yahoo.com/neo/groups/unison-users/conversations/topics/11660>

I am asking for such a feature because I want to have the opinion of
the maintainers of that project.
I am aware of the current status (not in active developend) and of the
less resources of the project.
I don't ask about if you (the maintainers) have the resources to
implement that. I am just asking if you think it is a good idea and you
would accept a patch/pull-request. Maybe you hate this idea and would
never accept that feature in an official unison release. It is
important do know for any developer who try to implement that.

The Feature:
Unison should check if the backup-files have a corrosponding original
file. If the original file doesn't exists anymore something should be
done (e.g. delete it). This could be depended on the age of the
bak-file, on the size of the complete backup-directorie, etc.
This would prevend the user for wasting storage space. When the user
delete an original file the backup-file is still there. Most of the use
cases it is in a hidden directory. So the user never think about that.
Using unison for some years this would produce a lot of files.

In my opinion and habit of developing software this FR could be a
bug-report, too.
The use of bak-files is not fully implemented. Unison create them
automaticly but doesn't take care of there deletion. In german we say:
"When you say A you have to say B, too."

Minimaly there should be a big-red-bold-hint in the manual about the
possible excessive use of storage space the user should take care about
himself.

So what is your opinion about this feature and would you accept it no
matter about your own resources?
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