New: Enhance stock management: Introduce tag to define if a subproduct

htdocs/fourn/class/fournisseur.product.class.php

htdocs/install/mysql/migration/3.7.0-3.8.0.sql

htdocs/install/mysql/tables/llx_product_association.sql

htdocs/install/mysql/tables/llx_stock_mouvement.sql

htdocs/langs/en_US/stocks.lang

htdocs/product/class/product.class.php

htdocs/product/composition/card.php

htdocs/product/stock/class/mouvementstock.class.php

htdocs/product/stock/mouvement.php

htdocs/product/stock/product.php