ed menus inside of `core-menu` elements.

    <core-menu selected="0">
    
      <core-submenu icon="settings" label="Topics">
        <core-item label="Topic 1"></core-item>
        <core-item label="Topic 2"></core-item>
      </core-submenu>
      
      <core-submenu icon="settings" label="Favorites">
        <core-item label="Favorite 1"></core-item>
        <core-item label="Favorite 2"></core-item>
        <core-item label="Favorite 3"></core-item>
      </core-submenu>
      
    </core-menu>
    
