Implement delete_in_overlay option for menu node delete action

Hi,

I try to implement deleteInOverlay action based on createInOverlay and editInOverlay.
This following the next PR:
 - sonata-project/SonataDoctrinePhpcrAdminBundle#243
 - symfony-cmf/MenuBundle#174

Previous PR was done https://github.com/symfony-cmf/TreeBrowserBundle/pull/34 but nothing was merged.
I proposed this one pending a better way.