Introduce predefined path in templates xml file for custom fields

#### Introduces the same functionality as PR #4294

How to test:
edit template xml [isis.xml] file adding 
```<field name="compile" type="compile" label="TPL_ISIS_COMPILE_LABEL" />``` under a fieldset
create a folder fields and a file in that fodder named ```compile.php```
and paste this code:
```
<?php
/**
 * @package     Joomla.Administrator
 * @subpackage  Templates.isis
 *
 * @copyright   Copyright (C) 2005 - 2014 Open Source Matters, Inc. All rights reserved.
 * @license     GNU General Public License version 2 or later; see LICENSE.txt
 */

defined('_JEXEC') or die;

jimport('joomla.filesystem.file');

class JFormFieldCompile extends JFormField
{
	protected function getInput()
	{

		$app         = JFactory::getApplication();
		$template    = $app->getTemplate(true);
		$params      = $template->params;
		$jinput      = $app->input;
		$compile     = 0;
		$compile     = $jinput->get('compileless');
		$currentpath = dirname(realpath(__DIR__));
		$pageurl     = str_replace('&amp;compileless=1', '', JURI::getInstance());

		if ($compile)
		{
			// Load the RAD layer to use its LESS compiler
			if (!defined('FOF_INCLUDED'))
			{
				require_once JPATH_LIBRARIES . '/fof/include.php';
			}

			$less = new FOFLess;
			$less->setFormatter(new FOFLessFormatterJoomla);

			//update variables.less
			$less->setVariables(array(
				"templateColor"    => $params->get('templateColor'),
				"headerBackground" => $params->get('headerColor'),
				"sidebarColor"     => $params->get('sidebarColor')
			));

			try
			{
				$less->compileFile($currentpath . '/less/template.less', $currentpath . '/css/template.css');
			} catch (Exception $e)
			{
				$app->enqueueMessage($e->getMessage(), 'error');
			}
		}

		return '<button onclick="window.location.href=\'' . $pageurl . '\'+\'&amp;compileless=1\'" class="btn btn-primary" type="button">Compile Less</button>';
	}

}
```
Visiting the admin -> templates -> isis [style] you should see the compile button. Press it an you will recompile the template.css