[#33962] Use a proxy function to close modals.

Because otherwise, we will always be stuck with SqueezeBox/Mootools.

Tracker: http://joomlacode.org/gf/project/joomla/tracker/?action=TrackerItemEdit&tracker_item_id=33962&start=0
It was added with https://github.com/joomla/joomla-cms/pull/4002 by @nikosdion.