Delete parent page, child pages cannot be found anymore

This PR fixes a bug by making sure that CMSSiteTreeFilters passes the correct method for
counting the number of children into SiteTree::CMSTreeClasses() so the correct CSS / JS
classes is set so the they can be opened in the UI.

PR for CMS https://github.com/silverstripe/silverstripe-cms/pull/1056
Issue:  https://github.com/silverstripe/silverstripe-cms/issues/1049