Allow for inheritance of Controller::$_render property within subclasses

I'd like the be able to do the following in a few controllers of mine which utilize an alternative layout:

	class DashboardController extends \lithium\action\Controller {
		
		/**
		 * Specify dashboard layout by default
		 *
		 */
		protected $_render = array('layout' => 'dashboard');
	
		[...]
	}

However, when doing this, all of the other default attributes of the `$_render` property become undefined unless I lift the entire array from the `\lithium\action\Controller` class.

So I'm left with doing this instead:

	class DashboardController extends \lithium\action\Controller {
	
		/**
		 * Specify dashboard layout by default
		 *
		 */
		public function __construct(array $config = array()) {
			$defaults = array('render' => array('layout' => 'dashboard'));
			parent::__construct($config + $defaults);
		}
	
		[...]
	}

This PR aims to make inheritance within the $_render property simpler by inheriting and merging defaults from all parent classes similar to how `\lithium\data\Model` handles inheritance in the $_meta property.