Make newly created object available to callback closure function

Then within the callback closure function, extra properties can be set (optionally) on the newly created object if necessary.

Example usage:

		// Closure for getting tags, including newly created one
		$blogitem = $this;
		$tagsource = function($newtag = null) use($blogitem) { 
			// link created FilterableTag to Parent of this page
			if($newtag){
				$newtag->ItemHolderID = $blogitem->Parent()->ID;
				$newtag->write();
			}
			// return all Tags (including new one)
			return $blogitem->Parent()->Tags()->map()->toArray();
		};
		// Add Categories field
		$tagsList = ListboxField::create(
				"Tags", _t("FilterableArchive.Tags", "Tags"), 
				$tagsource()
			)->setMultiple(true);
		$tagsList->useAddNew('Tag', $tagsource);
		
		$fields->insertBefore($tagsList, "Content");