title: Magento - add an option in Category's Custom Design
body: <p>In <code>Catalog &gt; Manage Categories</code>
  Under <code>Custom Design</code> tab, I want to add a new field just below <code>Apply to Products</code>.</p>
  
  <p>Upon searching I found a php file</p>
  
  <pre><code>app\code\core\Mage\Catalog\controllers\CategoryController.php
  </code></pre>
  
  <p>but I cannot find where exactly is it creating those tabs <code>Custom Design</code> and how is it adding the fields.</p>
  
  <p>Since it is not a good idea to change admin panel files, which class do I need to inherit from to create an extension that adds a field in the edit category ?</p>
  
  <p>I have found it is loading fields in</p>
  
  <pre><code>app\code\core\Mage\Adminhtml\Block\Catalog\Category\Tab\Design.php
  </code></pre>
  
  <p>and it seems field are defined in database</p>
  
  <pre><code>catalog_eav_attribute` table and `eav_attribute table
  </code></pre>
  
  <p>I added a new field in database but no success. How do I build this extension?</p>