Update toolbarButtons list

Add 'participants-pane' option

config.js

     // - if `toolbarButtons` is undefined, we fallback to enabling all buttons on the UI
     // toolbarButtons: [
     //    'microphone', 'camera', 'closedcaptions', 'desktop', 'embedmeeting', 'fullscreen',
-    //    'fodeviceselection', 'hangup', 'profile', 'chat', 'recording',
+    //    'fodeviceselection', 'hangup', 'profile', 'participants-pane', 'chat', 'recording',
     //    'livestreaming', 'etherpad', 'sharedvideo', 'shareaudio', 'settings', 'raisehand',
     //    'videoquality', 'filmstrip', 'invite', 'feedback', 'stats', 'shortcuts',
     //    'tileview', 'select-background', 'download', 'help', 'mute-everyone', 'mute-video-everyone', 'security'