Panel layout extended

As discussed with @contao/workgroup-core here is my new pull request.

It contains the panel_callback and the possibility to use the "filter" panel multiple times and assign the fields to them.

Testcase
---

Create a new module in ```system/modules/test``` and add the following code to ```/dca/tl_member.php```:

```
<?php

/**
 * Prepare both tests
 * Add a custom "foobar" panel to demonstrate the callback
 * Use "filter" multiple times to demonstrate how to assign them
 */
$GLOBALS['TL_DCA']['tl_member']['list']['sorting']['panelLayout'] = 'filter;sort,search;filter;foobar;limit;filter';

/**
 * panel_callback test
 */
$GLOBALS['TL_DCA']['tl_member']['list']['sorting']['panel_callback']['foobar'] = array('tl_member_test', 'addFoobarPanel');

class tl_member_test extends Backend
{
	public function addFoobarPanel(DataContainer $dc)
	{
		return '<div class="tl_sorting tl_subpanel"><strong>TEST</strong></div>';
	}
}

/**
 * Filter assignment test
 */
$GLOBALS['TL_DCA']['tl_member']['fields']['country']['filter'] = 2;
$GLOBALS['TL_DCA']['tl_member']['fields']['groups']['filter'] = 3;
```