org-colview: Fix age related summary
* lisp/org-colview.el (org-columns-time): Rename to...
(org--columns-time): ... this.
(org-columns--fractional-duration-re): New variable.
(org-columns-edit-value):
(org-columns):
(org-columns-compute-all):
(org-agenda-columns): Use new variable.
(org-columns-compile-map): Remove unneeded "calc" attribute.
(org-columns-number-to-string):
(org-columns-string-to-number): Fix ages set as a duration (e.g., "1d
12h"). Compare ages as seconds, not number of days.
* doc/org.texi (Column attributes): Document syntax for "age".