> 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

Here is an example where on the first level of outlines there
are project names, and on the second level are mode names: