authPrompt: emit prompted when given a message

Some pam modules prompt without expecting the user to type
an answer back (e.g. "Please swipe finger").  We need to
emit prompted in this case too, so the the dialog will get shown.

https://bugzilla.gnome.org/show_bug.cgi?id=683437

js/gdm/authPrompt.js

@@ -222,6 +222,7 @@ const AuthPrompt = new Lang.Class({
 
     _onShowMessage: function(userVerifier, message, styleClass) {
         this.setMessage(message, styleClass);
+        this.emit('prompted');
     },
 
     _onVerificationFailed: function() {