Added expressions for product/service prices

htdocs/core/boxes/box_produits.php

htdocs/core/boxes/box_produits_alerte_stock.php

htdocs/core/class/html.form.class.php

htdocs/fourn/ajax/getSupplierPrices.php

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

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

htdocs/install/mysql/tables/llx_product.sql

...cs/install/mysql/tables/llx_product_fournisseur_price.sql

htdocs/install/mysql/tables/llx_product_price.sql

htdocs/langs/en_US/products.lang

htdocs/product/class/priceparser.class.php

htdocs/product/class/product.class.php

htdocs/product/expression.php

htdocs/product/fournisseurs.php

htdocs/product/index.php

htdocs/product/price.php