Bugfix about using the calendar on a separate frame.

`org-read-date' was loosing the focus when the calendar was displayed on
a separate frame.  This patch by Robert Goldman solves it by introducing
a new macro `org-save-frame-excursion' which preserve the frame focus.

See this thread: http://article.gmane.org/gmane.emacs.orgmode/15528

(This fixes the issue I was trying to fix in a previous commit.)

lisp/org.el

@@ -12031,6 +12031,14 @@ So these are more for recording a certain time/date."
 (defvar org-read-date-history nil)
 (defvar org-read-date-final-answer nil)
 
+(defmacro org-save-frame-excursion (&rest body)
+  "Eval BODY and return to the currently selected frame."
+  (let ((frame-var (gensym "FRAME")))
+    `(let ((,frame-var (selected-frame)))
+       (unwind-protect
+           (progn ,@body)
+         (select-frame-set-input-focus ,frame-var)))))
+
 (defun org-read-date (&optional with-time to-time from-string prompt
 				default-time default-input)
   "Read a date, possibly a time, and make things smooth for the user.
@@ -12109,6 +12117,7 @@ user."
      (org-read-date-popup-calendar
       (save-excursion
 	(save-window-excursion
+	  (org-save-frame-excursion
 	   (calendar)
 	   (calendar-forward-day (- (time-to-days def)
 				    (calendar-absolute-from-gregorian
@@ -12179,7 +12188,7 @@ user."
 	       (use-local-map old-map)
 	       (when org-read-date-overlay
 		 (org-delete-overlay org-read-date-overlay)
-		(setq org-read-date-overlay nil)))))))
+		 (setq org-read-date-overlay nil))))))))
      
      (t ; Naked prompt only
       (unwind-protect
@@ -12199,6 +12208,7 @@ user."
 		  (nth 5 final) (nth 4 final) (nth 3 final)
 		  (nth 2 final) (nth 1 final))
 	(format "%04d-%02d-%02d" (nth 5 final) (nth 4 final) (nth 3 final))))))
+
 (defvar def)
 (defvar defdecode)
 (defvar with-time)