[*] BO : Add block "override_form_extra"

Add block "override_form_extra" in list_header.tpl


{if !$simple_header}

<form method="post" action="{$action}" class="form">



	{block name="override_form_extra"}{/block}




In AdminXxxxController, If we display list which are filtered by "variable" like

index.php?controller=AdminXxxx&variable=val&token=...

When using pagination or columns filters, the controller is called without our variable.

There are two methods to retrieve our variable: cookie or form Post.

To use the second opportunity, we need to put hidden input "variable" in the form.

So we need à block in the form to override.
Merge pull request #389 from ccauw/patch-5

[*] BO : Add block "override_form_extra"