title: Icons available for ExtJS's Panel's "tool" buttons
body: <p>How can I set icon used in my Panel's "Title Bar"? Maybe I need to add an image myself, but if so I suppose I need to define or configure that somewhere? </p>
  
  <pre><code>{
      xtype: 'treepanel',
      title: 'Projects',
      width: 200,
      store: Ext.data.StoreManager.lookup('projects'),
      tools: [
          {
              type: 'add', // this doesn't appear to work, probably I need to use a valid class
              tooltip: 'Add project',
              handler: function() {
                  console.log('TODO: Add project');
              }
          },
          ...
      ]
  },
  </code></pre>