Set only tree mode on document inside One2Many

Those list are there for reporting and quick information. But not for working
on the document because buttons are disable on the first modification. The
relate actions are the proper way to work on the documents.

issue7823
review53461002

view/sale_form.xml

@@ -5,7 +5,7 @@ this repository contains the full copyright notices and license terms. -->
     <xpath
         expr="/form/notebook/page[@name='shipments']/field[@name='shipments']"
         position="after">
-        <field name="drop_shipments" colspan="4"/>
+        <field name="drop_shipments" colspan="4" mode="tree"/>
     </xpath>
     <xpath expr="/form/notebook/page[@id='other']/field[@name='shipment_method']"
         position="after">