Order products in categories using term_relationships order

For issue #310

First pass at using `{$wpdb->term_relationships}.term_order` for ordering product within categories.

This currently work with latest master branch but only for v1 theme engine.

It basically only uses the 'term_order' if querying a product category, otherwise it falls back to 'menu_order'.

Both menu_order and term_order are set when re-ordering products for back-compatibility.