Support RFB bell

The protocol handling was already there. What was missing was
hooking it up to a sound file in the user interface.

app/sounds/CREDITS

@@ -0,0 +1,4 @@
+bell
+        Copyright: Dr. Richard Boulanger et al
+        URL: http://www.archive.org/details/Berklee44v12
+        License: CC-BY Attribution 3.0 Unported

app/styles/base.css

@@ -409,6 +409,10 @@ html {
   color:green;
 }
 
+#noVNC_bell {
+  display: none;
+}
+
 /* ----------------------------------------
  * Media sizing
  * ----------------------------------------

app/ui.js

@@ -205,6 +205,7 @@ var UI;
                                   'onUpdateState': UI.updateState,
                                   'onXvpInit': UI.updateXvpButton,
                                   'onClipboard': UI.clipboardReceive,
+                                  'onBell': UI.bell,
                                   'onFBUComplete': UI.initialResize,
                                   'onFBResize': UI.updateViewDrag,
                                   'onDesktopName': UI.updateDocumentTitle});
@@ -1280,6 +1281,12 @@ var UI;
             document.title = name + " - noVNC";
         },
 
+        bell: function(rfb) {
+            if (WebUtil.getConfigVar('bell', 'on') === 'on') {
+                document.getElementById('noVNC_bell').play();
+            }
+        },
+
         //Helper to add options to dropdown.
         addOption: function(selectbox, text, value) {
             var optn = document.createElement("OPTION");