Fixes: dragging sortable table rows into nested tables.

If you have nested tables inside the one-to-many table cells, items:
'tr' adds sortable behavior to nested table rows as well.  This, in
particular, means that you can drag top-level table rows into the nested
table. '> tr' restricts sortable behavior to rows which are direct
children of the sortable table body.

See, for example: http://api.jqueryui.com/sortable/#option-items