Made products Orderable. Some shop managers would like to order their products by hand, this would probably be mostly used in some special cases (e.g. accessory for a product should be placed just after the related product).
The change requires a migration adding the "Product._order" column, it should be fine to set all orders to zero -- by adding a new default "_order previous_default_order" ordering, previous ordering may be preserved.
Also made a minor change to category page processor that allows using multi-column ordering like the above.
Merge branch 'feature/orderable-products' into develop