[FIX] product: find products of subcategories

When searching products for a specific category, display the products of the whole category subtree.

addons/product/product_view.xml

@@ -15,7 +15,7 @@
                     <filter string="Consumable" name="consumable" icon="terp-accessories-archiver" domain="[('type','=','consu')]" help="Consumable products"/>
                     <separator/>
                     <filter string="Can be Sold" name="filter_to_sell" icon="terp-accessories-archiver-minus" domain="[('sale_ok','=',1)]"/>
-                    <field name="categ_id"/>
+                    <field name="categ_id" filter_domain="[('categ_id', 'child_of', self)]"/>
                     <field string="Product Variant" name="product_variant_ids" filter_domain="['|', ('product_variant_ids.name','ilike',self), ('product_variant_ids.attribute_value_ids.name','ilike',self)]"/>
                     <field name="company_id"/>
                     <field name="pricelist_id" widget="selection" context="{'pricelist': self}" filter_domain="[]" groups="product.group_sale_pricelist"/> <!-- Keep widget=selection on this field to pass numeric `self` value, which is not the case for regular m2o widgets! -->