Add a menu entry to attach captured messages

Without this menu entry, one has to remember the command name which is
not convenient.  This entry also helps to discover the "capture" feature.

mu4e/mu4e-compose.el

@@ -397,6 +397,15 @@ simply executes `fill-paragraph'."
 		  :enable mu4e-compose-format-flowed)
       'sep)
 
+    (define-key-after
+      (lookup-key mml-mode-map [menu-bar Attachments])
+      [mu4e-compose-attach-captured-message]
+      '(menu-item "Attach captured message"
+		  mu4e-compose-attach-captured-message
+		  :help "Attach message captured in Headers View (with 'a c')"
+		  :visible (eq major-mode 'mu4e-compose-mode))
+      (quote Attach\ External...))
+
     ;; setup the fcc-stuff, if needed
     (add-hook 'message-send-hook
       (lambda () ;; mu4e~compose-save-before-sending