title: Property not readable on a inner class in a TreeModel
body: <p>Here i have a <code>ManagePageSource</code> which provide Model for TreeView using <code>mvvm</code> pattern of zk.<br>
  <code>ManagePageSource</code> has a inner class <code>Node</code> to hold treeNode Data.<br>
  <code>Node</code> has a name property and zk is not able to read it in zul file.</p>
  
  <pre><code>public class ManagePageSource implements Serializable{
  
      private static final long serialVersionUID = 1L;
  
      private TreeModel treeModel = null;
  
      public TreeModel getTreeModel() {
          TreeNode&lt;Node&gt;[] children = null;
              // create a node
          TreeNode&lt;Node&gt; node = new DefaultTreeNode&lt;Node&gt;(new Node("child"), children);
          // add child node to a node
          node.add(new DefaultTreeNode&lt;Node&gt;(new Node("child2")));
              // create treeModel and set nodes
          treeModel = new DefaultTreeModel(node);
  
          return treeModel;
      }
  
      // Inner class for holding tree data
      class Node implements Serializable{
          private static final long serialVersionUID = 1L;
          private String name = null;
          public Node(String name){
              this.name = name;
          }
          public String getName(){
              return name;
          }
          public void setName(String name){
              this.name = name;
          }
      }
  
  }
  </code></pre>
  
  <hr>
  
  <p>Manage.zul</p>
  
  <pre><code>&lt;window apply="org.zkoss.bind.BindComposer"
      viewModel="@id('vm') @init('ManagePageSource')"&gt;
      &lt;tree model="@load(vm.treeModel)"&gt;
          &lt;treecols&gt;
              &lt;treecol label="Path" /&gt;
          &lt;/treecols&gt;
          &lt;template name="model"&gt;
              &lt;treeitem&gt;
                  &lt;treerow&gt;
                      &lt;treecell label="@load(each.data.name)" /&gt;
                  &lt;/treerow&gt;
              &lt;/treeitem&gt;
          &lt;/template&gt;
      &lt;/tree&gt;
  &lt;/window&gt;
  </code></pre>
  
  <p>error stack</p>
  
  <pre><code>&gt;&gt;org.zkoss.zel.PropertyNotFoundException: Property 'name' not readable on type ManagePageSource$Node
  &gt;&gt;  at org.zkoss.zel.BeanELResolver$BeanProperty.read(BeanELResolver.java:409)
  &gt;&gt;  at org.zkoss.zel.BeanELResolver$BeanProperty.access$000(BeanELResolver.java:320)
  </code></pre>
  
  <p>Why name is not readable here ?</p>