Delete parent page, child pages cannot be found anymore

[dont merge]  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
Only reason for this is so that it can pass the numChildrenMethod to SiteTree:: CMSTreeClasses() that passes it back to Hierarchy::markingClasses().. *sigh* See the framework PR