title: Query for child component with a specific xtype
body: <p>Is there a method to query for a child component of xtype 'yourxtype' ?
  For example you have a panel, and inside this panel is a custom xtype located on a toolbar.</p>
  
  <p>The findParentByType works great for looking up, but there is not method like findChildByType.
  And the down method works only on elements. The Ext.ComponentQuery is a singleton, but not for xtypes? 
  Is there another way to query from a component down?</p>