Add inventory field on tree view of line

Without the inventory field, the inventory_state is not updated with the
on_change_with and so the lines are readonly.

issue6281
review32041002

view/inventory_line_tree.xml

@@ -2,6 +2,7 @@
 <!-- This file is part of Tryton.  The COPYRIGHT file at the top level of
 this repository contains the full copyright notices and license terms. -->
 <tree editable="bottom">
+    <field name="inventory"/>
     <field name="product"/>
     <field name="expected_quantity"/>
     <field name="quantity"/>