Adds the ability for single level grouping to the Listbox.

<?xml version="1.0" encoding="UTF-8"?>
<zk>
    Listbox support Grouping    
    <listbox id="listbox" width="250px">    
        <listhead sizable="true" id="h">        
            <listheader id="h1" label="name" sort="auto" />            
            <listheader id="h2" label="gender" sort="auto" />            
        </listhead>        
        <listgroup id="gp1" open="false">        
            <listcell label="Group1"/>            
            <listcell label="Group2"/>            
        </listgroup>        
        <listitem>        
            <listcell label="a Mary" />            
            <listcell label="a FEMALE" />            
        </listitem>        
        <listitem>        
            <listcell label="b Mary" />            
            <listcell label="b FEMALE" />            
        </listitem>        
        <listitem id="li1">        
            <listcell label="c Mary1" />            
            <listcell label="c FEMALE1" />            
        </listitem>        
        <listitem>        
            <listcell label="d Mary" />            
            <listcell label="d FEMALE" />            
        </listitem>        
        <listitem>        
            <listcell label="e John" />            
            <listcell label="e MALE" />            
        </listitem>        
        <listgroup id="g2" label="Grouping 2" />        
        <listitem>        
            <listcell label="Jane" />            
            <listcell label="FEMALE" />            
        </listitem>        
        <listitem>        
            <listcell label="Henry" />            
            <listcell label="MALE" />            
        </listitem>        

    </listbox>    
</zk>