Ensure product type stays in sync

[pcfreak30]

In the scenario that a product data store does no extend the custom datastores, it needs to hook woocommerce_product_type_changed to sync the changed product type to the custom products table.

It is unknown if there will be any side effects but it gives at least partial support to woo commerce extensions with their own datastores that extend WC_Product_Data_Store_CPT.