Add default View tab to the tab list when there are other subpages (based on commit d1c33ef5). Also fixed the logic to determine if a subpage has content, since check_markup() returns n/a when the string is empty

Make sure that default_view_page is set on all subpages, to prevent unset index notices. Copied from commit 66e10af3

Load subpages by weight, so that they are ordered correctly, and don't return NULL for a page output (results in a blank page if you do that)