title: xtype in Extjs4 creates same id
body: <p>In extjs4 if I have a button as follows: </p>
  
  <pre><code>Ext.define('Pandora.view.MyButton', {
  extend: 'Ext.button.Button',
  alias: 'widget.mybutton',
  text: 'click me',
  id: 'butid'
  </code></pre>
  
  <p>});</p>
  
  <p>If I use it in viewport as follows: </p>
  
  <pre><code>                    {
                        xtype: 'panel',
                        id: 'panelid',
                        title: 'One tab',
                        html: '&lt;p&gt;what is this &lt;/p&gt;'
                      },
                      {
                        xtype: "mybutton"
  
                      },
                      {
                        xtype: "mybutton"
                      }
  </code></pre>
  
  <p>This will create two buttons with same id (which is not W3C recommended), and it could produce weird behavior. In order to reuse same button multiple times in application is it recommended to use xtype without providing id (In which case Extjs will create its own).</p>