(ibuffer-update-mode-name): Substitute "view time" instead of

(ibuffer-update-mode-name): Substitute "view time" instead of
"recency" for clarity.
(ibuffer-compile-format): Document more.  Handle new "summarizer"
columns.
(ibuffer-fontify-region-function): Ditto.
(ibuffer-insert-buffer-line): Ditto.
(ibuffer-map-lines): Ditto.
(ibuffer-insert-buffers-and-marks): Ditto.
(ibuffer-update-title-and-summary): Renamed from
`ibuffer-update-title'.  Handle "summarizer" columns.
(ibuffer-clear-summary-columns): New function.