Moved class to other file. Fix issue #4551

The file https://github.com/joomla/joomla-cms/blob/staging/administrator/includes/toolbar.php breaks the principle of having 1 class per file: JToolbarHelper and JSubMenuHelper

@chrisdavenport suggested that we could maybe fix it by removing JSubMenuHelper and including it from JToolbarHelper file. That way we will not break b/c. How does it sound?

see #4551