org.el (org-sort-entries): Allow to sort by clocking time

* org.el (org-sort-entries): Allow to sort by clocking time.

Thanks to Noah Slater for this idea.

lisp/org.el

@@ -8683,20 +8683,21 @@ a time stamp, by a property, by priority order, or by a custom function.
 
 The command prompts for the sorting type unless it has been given to the
 function through the SORTING-TYPE argument, which needs to be a character,
-\(?n ?N ?a ?A ?t ?T ?s ?S ?d ?D ?p ?P ?o ?O ?r ?R ?f ?F).  Here is the
-precise meaning of each character:
+\(?n ?N ?a ?A ?t ?T ?s ?S ?d ?D ?p ?P ?o ?O ?r ?R ?f ?F ?k ?K).  Here is
+the precise meaning of each character:
 
-n   Numerically, by converting the beginning of the entry/item to a number.
 a   Alphabetically, ignoring the TODO keyword and the priority, if any.
-o   By order of TODO keywords.
-t   By date/time, either the first active time stamp in the entry, or, if
-    none exist, by the first inactive one.
-s   By the scheduled date/time.
-d   By deadline date/time.
 c   By creation time, which is assumed to be the first inactive time stamp
     at the beginning of a line.
+d   By deadline date/time.
+k   By clocking time.
+n   Numerically, by converting the beginning of the entry/item to a number.
+o   By order of TODO keywords.
 p   By priority according to the cookie.
 r   By the value of a property.
+s   By scheduled date/time.
+t   By date/time, either the first active time stamp in the entry, or, if
+    none exist, by the first inactive one.
 
 Capital letters will reverse the sort order.
 
@@ -8775,8 +8776,8 @@ When sorting is done, call `org-after-sorting-entries-or-items-hook'."
     (unless sorting-type
       (message
        "Sort %s: [a]lpha  [n]umeric  [p]riority  p[r]operty  todo[o]rder  [f]unc
-               [t]ime [s]cheduled  [d]eadline  [c]reated
-               A/N/P/R/O/F/T/S/D/C means reversed:"
+               [t]ime [s]cheduled  [d]eadline  [c]reated  cloc[k]ing
+               A/N/P/R/O/F/T/S/D/C/K means reversed:"
        what)
       (setq sorting-type (read-char-exclusive))
 
@@ -8794,6 +8795,7 @@ When sorting is done, call `org-after-sorting-entries-or-items-hook'."
 				       (mapcar 'list (org-buffer-property-keys t))
 				       nil t))))
 
+    (when (member sorting-type '(?k ?K)) (org-clock-sum))
     (message "Sorting entries...")
 
     (save-restriction
@@ -8828,6 +8830,8 @@ When sorting is done, call `org-after-sorting-entries-or-items-hook'."
 	     (if (looking-at org-complex-heading-regexp)
 		 (funcall case-func (org-sort-remove-invisible (match-string 4)))
 	       nil))
+	    ((= dcst ?k)
+	     (get-text-property (point) :org-clock-minutes))
 	    ((= dcst ?t)
 	     (let ((end (save-excursion (outline-next-heading) (point))))
 	       (if (or (re-search-forward org-ts-regexp end t)