mu4e: display signers in signature header

Display the signers in the signature headers, so users can easily very
that a verified signed message was indeed signed by the expected sender.

We can't check using 'From:' automatically as that gives too many false
negatives.

mu4e/mu4e-view.el

@@ -460,6 +460,11 @@ add text-properties to VAL."
 	    (remove-if 'null
 	      (mapcar (lambda (part) (mu4e-message-part-field part :signature))
 		parts)))
+	  (signers
+	    (mapconcat 'identity
+	      (remove-if 'null
+		(mapcar (lambda (part) (mu4e-message-part-field part :signers))
+		  parts)) ", "))
 	  (val (when verdicts
 		 (mapconcat
 		   (lambda (v)
@@ -474,7 +479,7 @@ add text-properties to VAL."
 			       (mu4e-view-verify-msg-popup
 				 (button-get b 'msg))))
 		   (buffer-string))))
-	  (val (when val (concat val " (" btn ")"))))
+	  (val (when val (concat val signers " (" btn ")"))))
     (mu4e~view-construct-header :signature val t)))
 
 (defun mu4e~view-construct-decryption-header (msg)