(ibuffer-filtering-groups): Renamed to

(ibuffer-filtering-groups): Renamed to
`ibuffer-filter-groups'.  All users updated.
(ibuffer-show-empty-filter-groups): New variable.
(ibuffer-saved-filter-groups): New variable.
(ibuffer-maybe-save-stuff): Renamed from
`ibuffer-maybe-save-saved-filters'.  Callers updated.  Handle
`ibuffer-saved-filter-groups'.
(ibuffer-hidden-filtering-groups): Renamed to
`ibuffer-hidden-filter-groups'.
(ibuffer-filter-group-kill-ring): New variable.
(ibuffer-save-with-custom): Doc fix.
(ibuffer-set-filter-groups-by-mode): New function.
(ibuffer-clear-filter-groups): New function.
(ibuffer-current-filter-groups-with-position): Renamed from
`ibuffer-current-filter-groups'.  Callers updated.
(ibuffer-kill-filter-group): New function.
(ibuffer-kill-line): New function.
(ibuffer-yank): New function.
(ibuffer-save-filter-groups): New function.
(ibuffer-delete-saved-filter-groups): New function.
(ibuffer-switch-to-saved-filter-groups): New function.
(ibuffer-mark-on-buffer): Add optional arguments for adding a
specific mark, and marking on a specific group.