BUG: 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 framework https://github.com/silverstripe/silverstripe-framework/pull/3309

Issue link: https://github.com/silverstripe/silverstripe-cms/issues/1049
I'm unsure if this is a performance killer, or by casting it to map it actually doesn't create a new instance of sitetree.