org.el: Enable recursive minibuffers in `org-completing-read'.

org.el: (org-completing-read): Enable recursive minibuffers and add
the `C-c !' key in the minibuffer local map.

Thanks to Skip Collins for the idea and to Nick Dokos for the
implementation.

lisp/org.el

@@ -9159,10 +9159,12 @@ Use TAB to complete link prefixes, then RET for type-specific completion support
 
 (defun org-completing-read (&rest args)
   "Completing-read with SPACE being a normal character."
-  (let ((minibuffer-local-completion-map
+  (let ((enable-recursive-minibuffers t)
+	(minibuffer-local-completion-map
 	 (copy-keymap minibuffer-local-completion-map)))
     (org-defkey minibuffer-local-completion-map " " 'self-insert-command)
     (org-defkey minibuffer-local-completion-map "?" 'self-insert-command)
+    (org-defkey minibuffer-local-completion-map (kbd "C-c !") 'org-time-stamp-inactive)
     (apply 'org-icompleting-read args)))
 
 (defun org-completing-read-no-i (&rest args)