bug#70150: Better groups for Buffer-menu-group-by

[Juri Linkov]

close 70150 30.0.50
thanks

>> Here is a better grouping for Buffer-menu-group-by-mode
>> that uses the existing mode categorization in
>> mouse-buffer-menu-mode-groups.
>
> This is the final patch that finishes the remaining features:
>
> 1. allows multi-level outlines
> 2. allows an entry to be in multiple groups
> 3. allows sorting of groups

Now pushed and closed.