New: add vat rate in supplier price

htdocs/core/ajax/vatrates.php

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

htdocs/core/lib/ajax.lib.php

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

htdocs/install/mysql/migration/3.1.0-3.2.0.sql

...nstall/mysql/tables/llx_product_fournisseur_price.key.sql

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

htdocs/product/fournisseurs.php

htdocs/societe/soc.php