* mu4e: enable showing the message-id in the view (and even the headers)

mu4e/mu4e-headers.el

@@ -426,7 +426,7 @@ if provided, or at the end of the buffer otherwise."
 		  (mu4e~headers-thread-prefix (mu4e-message-field msg :thread))
 		  ;;  "["(plist-get (mu4e-message-field msg :thread) :path) "] "
 		  val))
-	      ((:maildir :path) val)
+	      ((:maildir :path :message-id) val)
 	      ((:to :from :cc :bcc) (mu4e~headers-contact-str val))
 	      ;; if we (ie. `user-mail-address' is the 'From', show
 	      ;; 'To', otherwise show From

mu4e/mu4e-vars.el

@@ -558,6 +558,11 @@ I.e. a message with the draft flag set."
 	 :shortname "List"
 	 :help "Mailing list for this message"
 	 :sortable nil))
+     (:message-id .
+       ( :name "Message-Id"
+	 :shortname "MsgID"
+	 :help "Message-Id for this message"
+	 :sortable nil))
      (:path .
        ( :name "Path"
 	 :shortname "Path"

mu4e/mu4e-view.el

@@ -187,6 +187,8 @@ messages - for example, `mu4e-org'."
 	      (mu4e~view-construct-header field (mu4e-display-size fieldval)))
 	    (:mailing-list
 	      (mu4e~view-construct-header field fieldval))
+	    (:message-id
+	      (mu4e~view-construct-header field fieldval))
 	    ;; attachments
 	    (:attachments (mu4e~view-construct-attachments-header msg))
 	    ;; pgp-signatures